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AMERICAN SOCIETY OF CIVIL ENGINEERS. 


THE CAUSE AND PREVENTION 

OF THE 

DECAY OF BUILDING STONE. 


— BY — 


THOMAS EGLESTON, Ph.D., M. Am. Soc. 0. E. 



















~r 


O 


5 


AMERICAN SOCIETY OF CIVIL ENGINEERS, 


INSTITUTED 1852. 


TRANSACTIONS. 


Note.—T his Society is not responsible, as a body, for the facts and opinions advanced in 
any of its publications. 


p-Ct/y 


(Yol. XY.-, 1886.) 


THE CAUSE AND PREVENTION OF THE DECAY 
OF BUILDING STONE. 


By Thomas Egleston, Pli.D., M. Am. Soc. C. E. 
Read June 24th, 1885. 


In the years 1858-59, I had occasion to pass several months in the 
City of Rodez, the capital and also the cathedral town of the Depart¬ 
ment of Aveyron, in France. The cathedral was on the line of the Hugue¬ 
not wars. It showed in every part the mistaken zeal of those religion¬ 
ists, much of whose energy was expended in destroying what was most 
beautiful. The main doors were a succession of diminishing arches 
lined with beautifully carved figures of saints in high relief, but every 
projecting piece that could be reached had been battered with a ham¬ 
mer, so that there was not a head, a leg or an arm left on the hundreds 
of figures that formerly ornamented these doors. The cathedral foun¬ 
dations were laid in the eighth century. The building was finished in the 
last. It is constructed of red sandstone, very like the stone so much 
used in this city, and is the most prominent building in the vicinity. 
I had not passed it many times before I noticed that the stone of which 
it was built was subject to serious decay. It was not long before my at¬ 
tention was attracted to the fact that while the lower part of the church, 








2 


which was completed previous to the fifteenth or sixteenth century, 
had the mouldings and carvings which were left by the Huguenots per¬ 
fectly sharp; all of the upper part, which was finished later, was almost 
a ruin. This was all the more remarkable, because the upper part was 
completed at a time when what might be called geometrical architecture 
was the fashion. There were geometrical staircases on every hand, 
staircases within staircases, lanterns supported on flying buttresses, 
and almost every other architectural puzzle that was in vogue in 
those days. It was not long before I noticed the fact that where 
the stone in any part of the building had a siliceous binding material, 
neither the mouldings nor the surface of the stone were decomposed; 
but in proportion as the siliceous binding material decreased in 
quantity, so also was the stone decomposed, until some parts of the 
church had actually fallen from decay, and others were in danger. Hav¬ 
ing occasion to pass the cathedral almost every day for four or five 
months during different years, I became greatly interested in the study 
of the decomposition that was taking place in the stone composing it, as 
also in all the other varieties used in the vicinity. Being obliged to 
return to America in the year 1860, I noticed the same process of de¬ 
composition going on in some of the brownstones of this city, and 
began almost at once to make some experiments with a view of arresting 
the decay, some of which were so successful that the building on which 
they were made had the progress of its decomposition almost entirely 
arrested, and it would have remained so for some years to come, if the 
front had not been taken down for alterations two years ago. 

In the year 1878, I noticed that the stone of which Trinity Church, 
in this city, is built, showed serious signs of decay. In the year 1880 
it became necessary to undertake a thorough investigation of the causes 
of this decay, in the hope of learning some means of checking it. The 
work was placed in my hands. As the examination could not but be a 
difficult one, I commenced it by making a careful study of all those 
parts of the church where the stone was decomposed. The fact became 
evident at once that in almost every place where decomposition had be¬ 
gun, it was immediately under a projecting piece which produced surfaces 
more or less horizontal. It was apparent at once that at least a part of 
the decay was due to a defect in the construction of the mouldings 
themselves, none of them being properly undercut, so that they do not 
shed rain as they should. In one or two instances imperfect stone was 


3 


used in the construction of the church, and in a very few places decom¬ 
position has been caused by the position occupied by the stone itself. 
These studies of the stone in place, and of its decomposition, lasted 
through several months. I examined the stone inside and outside of 
the church, as far as it was accessible, high up into the steeple, and 
became aware that there were used in its construction four different 
kinds of stone, and that the rate of decomposition was likely to be dif¬ 
ferent in each. 

After learning what these varieties of stone were, and the mode of 
decomposition of each, I selected for examination thirty specimens from 
all parts of the building, both inside and out, some of which were 
pieces apparently fresh, and others were scales of more or less thickness, 
which were easily separated from the stone. A careful physical, chem¬ 
ical and microscopical examination was made of all of these specimens. 
As the investigation was somewhat extended, and involved the complete 
review of the whole subject of the decay of building material, and as what 
has been supposed to be one of the most permanent building-stones of 
New York and other cities, is undergoing the same decay as other stones 
in this country, I obtained the permission of the corporation of Trinity 
Church to publish my results. I have thought that it would be of 
interest to present to this Society a short review of the whole subject 
of the decay of building-stone, as well as the investigation of the con¬ 
dition of this particular building. The matter is of all the greater 
interest at the present time, as not only some of the public buildings, 
but also monuments, which were designed to last for centuries, are, after 
only a very few years, showing serious signs of decay. 

When we take into consideration what may be found in the air of 
cities, it is not at all surprising that stones which contain any soluble 
material should have this soluble portion dissolved in the course of 
time. The air of any large city may contain carbonic acid, nitric acid, 
muriatic acid, several different varieties of organic acids, ozone and 
ammonia. On making some experiments, to ascertain the effect of 
solutions of carbonic acid dissolved in minute quantities in water, it was 
found that 1 428 parts of a saturated solution of carbonic acid in water 
were required to dissolve one part of carbonate of lime at zero Centi¬ 
grade; at 10 degrees Centigrade, it required only 1 136 parts of acid. 
At a much higher temperature a considerably larger quantity of lime 
was dissolved. It was also found that dolomite was not so readily 


4 


soluble in solutions containing carbonic acid as carbonate of lime. One 
part of artificial carbonate of magnesia is soluble in 150 to 300 parts of 
carbonic acid. The mineral magnesite is soluble in 5 071 parts of pure 
water at 15 degrees Centigrade, with no carbonic acid in it. Artificial 
carbonate of magnesia, containing three equivalents of water, is soluble 
in 48 parts. It is also a well-known fact that concentrated solutions 
dissolve proportionately less than very dilute ones. Carbonic acid is 
found in the purest air to the extent of about too of 1 per cent., and 
in the air of large cities to three times that amount. It is therefore 
not at all -wonderful that this action of solution should take place in 
building stone. It is known that nitric acid is generated in small 
quantities by lightning, and that muriatic acid is frequently found, to 
the amount of from 1 to l£ grains to the gallon of air, in localities 
situated near manufactories or near the sea. Organic acids are some¬ 
times found in the air in considerable quantities; and recent researches* 
have proved that these organic acids will attack minerals which are 
completely insoluble in ordinary acids, so that it has been proposed to 
substitute them entirely for the mineral acids in geological work; this is, 
perhaps, one of the reasons why the silicates in rocks are so easily 
absorbed by the plants, and will probably account for the ease of decom¬ 
position of the minerals, contained in the rocks which gelatinize with 
acids, and also for a considerable amount of the decomposition in the 
pyroxene and amphibole series. 

Messrs. Spring and Roland have recently communicated to the 
Belgian Academy a series of observations made upon the amount of 
carbonic acid contained in the atmosphere during a year. They give 
the result of 266 determinations made in the City of Liege, Belgium; 
on one side of which there is an industrial, on the other an agricultural 
district. The average amount of carbonic acid obtained in 10 000 parts 
of air was 5.1258 parts by weight and 3.3526 parts by volume. These 
gentlemen remark that this is more than the amount contained in the 
air of Paris, which is 4.831 parts by weight and 3.168 by volume. The 
larger amount at Liege is owing not only to the large iron-works there, 
but also to the fact that the city is surrounded by coal mines. To this the 
authors attribute the greater heat of the city, as it is well known that a 
small quantity of carbonic acid in the air causes the absorption and 
prevents the radiation of heat. They also attribute the cold weather 

* Annals New York Academy Science, Vol. I, p. 1. 




5 


of May to the diminution of the carbonic acid caused by the absorption 
of it by the growing leaves. Their observations show that a fall of snow 
will increase the amount to 3.761 by volume, and that in cloudy weather 
the amount was 3.571 parts, and that there was always a larger amount 
in winter than in summer. They also found that the quantity was 
diminished by high winds, but increased with a high barometer; all of 
which researches go to show that at the very time when the stone is 
most likely to be acted upon unfavorably—that is, in the winter—there 
is the greatest amount of carbonic acid in the air. In Liege there are 
some examples of the decomposition caused by the effect of gases, as 
striking as any that can be seen anywhere in Europe. 

The quantity of coal that is burned in the City of New York every 
year amounts to 4 500 000 tons. If we estimate that this coal contains, 
on an average, one-half of one per cent, of sulphur, which is an 
extremely low estimate, this would amount to about 35 pounds of ordi¬ 
nary sulphuric acid per ton of coal consumed, discharged into the air 
from the oxidation of the sulphur, and would amount in the course of 
a year to 78 750 tons of sulphuric acid. In this part of the country we 
burn anthracite, which gives off little else than carbonic acid, 
carbonic oxide and sulphur. But in regions where bituminous coal is 
used, ammoniacal constituents are given off as well, and these are cor¬ 
rosive. In order to ascertain how much sulphuric acid could be collected 
from the rainfall of the city, a funnel and bottle were placed out¬ 
side the window of the Metallurgical Laboratory of the School of 
Mines.* The funnel had an opening of 50 square centimeters. It 
was left exposed during 41 days. At the end of that time it contained 
65 cubic centimeters of water. This water was found to contain 4J- mil¬ 
ligrams of sulphuric acid. The experiment was not made with a view 
to catch all the sulphuric acid, but to see if some could be caught. 
Very many of the days were bright, and the quantity of acid gas thrown 
into the air must therefore have been many times that amount, although 
the quantity is very large and sufficiently accounts for the rapid decay 
going on in the brown stone of that building. It has been estimated 
that the value of a London winter’s smoke, including the carbon which 
is lost, as well as the other materials volatilized with it, amounts to 
$25 000 000. 

It does not appear that ordinary salt contained in the sea-air, or even 




* This experiment was made by Mr. J. B. Mackintosh. 





6 


a considerable amount of sea-spray, has any very deleterious effect upon 
stone-work, except in so far as it produces a certain amount of other 
alkaline substances by the action of the salt upon other bases, and in 
this way becomes, like all other dampness, an agent of decomposition. 
Almost the only effect of it observable on buildings is its attack on tin 
roofing, which requires a protection of paint near the sea, while in the 
interior of the country it will last for years without rusting. A very 
interesting paper, giving the results of observations made near the city 
on the quantity of chlorides contained in our rainfall in the year 1884, 
has been recently published.* It is of great interest in this connection 
as showing that rain water may be a fertilising as well as a very destruc¬ 
tive agent. 

It may be said, as a general rule, that stones are less valuable as they 
are more porous, for the reason that they are more likely to absorb water 
containing the gases which attack the ingredients, and also because the 
water, absorbed in freezing, by its expansion tends to disintegrate the 
stone. It has been said that the act of freezing is equivalent to the 
blow of a ten-ton hammer on every square inch of surface. Whether 
this be so or not, the continued expansion and contraction of a porous 
stone is quite sufficient to disintegrate it, and this disintegration will be 
all the greater as the stone contains more water. Stones that have 
already begun to decompose absorb a much larger quantity of water 
than those which are fresh from the quarry, independent of the quarry 
water which they contain, and hence the decay will be more rapid as it 
progresses. Professor Wigner, who examined the obelisk on the Thames 
embankment, found that the weathered surface of the obelisk absorbed 
six times the amount of water that was absorbed by the stone which had 
suffered no decomposition. This, of itself, though small in amount, 
would be sufficient, in a climate like ours, to produce serious disinte¬ 
gration in the stone. 

The builders of the early centuries seem to have understood the in¬ 
jurious action of water, whether as rain, hail or snow, upon their build¬ 
ings better than we do. Nothing is more striking than the shape of the 
lintels and sills of their great buildings, which are put in with a sharp 
incline downward. The architecture of these buildings was carefully 
studied, even in the minutest detail, and this is not at all strange, since 
these early masons and builders were a religious fraternity, who studied 


* School of Mines Quarterly, Vol. VI, p. 35. 




everything carefully, intending that their monuments, which are mostly 
churches, should be built to last for ages. 

It is very interesting in the history of a country to trace the effects 
of the building material which its people are in the habit of using, upon 
their methods of construction. In this country we began with wood, 
because that was the cheapest material and the most easily obtained. 
The forms that were adapted to wood, our architects have carried into 
stone, which is entirely unsuited for them, and hence the constant re¬ 
currence of flat surfaces, which retain the moisture and precipitate 
decomposition, which otherwise might be very slow. It is true that the 
old Dutch settlers here constructed their roofs to some extent with ref¬ 
erence to the great quantity of snow, which is the usual accompaniment 
of the winters of this climate; but their successors have everywhere 
adopted flat surfaces. It is not generally supposed that these are more 
deleterious to the stone in countries where there is snow and ice, than 
rain is in a country where there is only rain, but this is the fact; for 
where snow is allowed to lodge, the water running through it will concen¬ 
trate beneath on the surface of the stone, and even when the snow above 
is not melted, if there is any thawing at all, the snow will be constantly 
moist on the bottom against the stone. This moisture percolates through 
the stone. The decomposition under projecting window sills and on the 
lower side of balconies from this cause, can be noticed everywhere 
throughout our cities. 

There is another cause for the disintegration of stone, which, while 
it is not very powerful, contributes its share in weakening it, and that is 
the efflorescence upon or the formation within the stone itself of more or 
less soluble salts, which are generally sulphates. These, by exuding from 
the surface, and crystallizing both within and outside of the stone, must 
of necessity cause a pressure toward the outside, and thus, by producing 
more or less of a separation in the grains of the stone, tend to disinte¬ 
grate it. In the modern haste to finish things rapidly, and the anxiety 
to produce cheap materials, almost everything is done with a view to 
gain money or to save time, while thoroughness and durability are neg¬ 
lected. The lime of which the mortar is made is burned in direct con¬ 
tact with a refuse fuel, and this contains very frequently considerable 
quantities of iron pyrites, from which sulphates more or less acid are 
produced, which are retained in the lime, and as these sulphates are 
usually more or less soluble, they either effloresce from the mortar, 


8 


making it weak and causing it to dissolve out from the joints itself, or 
infiltrate through the stone and crystallize there, some of the substance 
coming to the outside as a white stain. 

There is, independent of this cause—imperfect work in the manufac¬ 
ture of brick—the formation of the sulphates of alumina from the pres¬ 
ence of pyrites in the clay. They exist in it much more frequently than 
is generally supposed. This would form an alkaline sulphate of alumina, 
containing either potash or soda, and tend to dissever or weaken the 
brick. Washing these efflorescences off with sulphuric acid, as is fre¬ 
quently done on brick buildings, or the cleaning of the surfaces of lime¬ 
stone in the same way, still further tends to increase the decomposition 
and consequent weakness, unless the stone or brick be immediately 
washed thoroughly with hot water, which, however, is never done. The 
use of acids to cleanse any stone or brick work is objectionable. There 
are a number of substances which can be used for this purpose, which 
produce no injurious effect on the brick or stone, and are more effectual, 
while being quite as cheap as any acid. In underground work the 
presence of any considerable amount of pyrites in clay should cause 
extra precaution to be taken with the structure. The brick-work of 
the first tunnel under the lake at Chicago was crushed in several places, 
and the tunnel shoved out of place in many more, by the decomposition 
of the pyrites in the clay, when the work was allowed to remain 
exposed to the air too long before filling the space between the brick¬ 
work and the clay. 

The corroding effect of sand carried by the wind at high velocity has, 
a very decided influence in producing the disintegration of the surfaces 
of building materials. This is evident on the surface of every building 
which is exposed to the dust of cities and swept by the prevailing winds. 
My attention was first called to it by being asked to study the reason of 
the defacement of the inscriptions on certain tombstones of historic in¬ 
terest some years ago, which seemed to be produced by no assignable 
cause. These tombs were placed just where they received the dust 
blown by the city winds, through an opening quite narrow, but where 
the action was constant whenever the winds were of any great velocity. 
To ascertain exactly what the effect of the power of sand might be 
when driven by wind, I undertook a series of experiments on the abra¬ 
sive power of sand, with the sand blast. These were made upon ordi¬ 
nary stones, commencing at first with the softer varieties. I very soon 


9 


fonnd that there was no stone of hardness sufficient to resist the power 
of the blast even for a few minutes. I then tried hard white cast- 
iron and hardened steel, but found that they had only a small re¬ 
sisting power, and wishing to ascertain exactly what the force of the 
blast was, in terms which would be readily understood, I commenced 
a series of experiments on the minerals which compose the scale of 
hardness. I very soon found that there was no use in experimenting 
on stones less hard than the topaz. The surface abrasion was so 
rapid that they were in a very short time reduced to powder, so that 
there was no opportunity of examining critically the effects of the 
blast. I very much regret that I did not have an opportunity of examin¬ 
ing the effect of the sand blast in regard to the toughness of materials acted 
on, as well as hardness; but the time during which I was permitted to use 
the blast was very limited, and I have never had the opportunity of con¬ 
tinuing my experiments. I however learned in the course of them, that on 
soft materials which were elastic, the effect of the blast was either reduced 
to zero or was very much diminished. It is well known to those using 
he sand blast that very hard substances may be entirely protected from 
the effect of the blast by using either a rubber coating or a film of some 
soft metal like tin, which, from its elasticity, will throw the grains off, 
while the harder surfaces will be abraded by them. This is no doubt 
the reason why soapstones and other varieties of very soft rocks, when 
they do not include within them minerals like pyrites, which, by their 
decomposition, cause them to disintegrate, have resisted atmospheric in¬ 
fluences for very long periods. 

I give below a table showing the results of these experiments on the 
minerals in the scale of hardness, giving the name of the mineral, the 
time that it was under abrasion, and the weight in grams of the quan¬ 
tity lost by abrasion. The specimens which were subjected to investiga¬ 
tion were necessarily small, as I had no idea when I began of the power 
of the blast. They, however, show conclusively the principle upon 
which the curious abrasions of rocks over large areas found in the far 
West, which are the admiration of the geologist and the wonder of the 
tourist, are produced: 


10 


Mineral. 

Time under 
the Blast. 

Quantity lost. 
Grams. 

Topaz (Gouted’Eau). 

. 1 minute 

1.9707 

“ pebble. 

. 1 

i « 

2.1499 

Emery from Chester, Mass. 

. 1 

i i 

4.9532 

Corundum from Delaware Co., Pa . 

• * 

it 

1.1698 

Black diamond. 

. 3 

a 

0.0372 

Black diamond. 

. 5 

i i 

0,0497 

Black diamond. 

. 8 

it 

0.0869 


The emery from Chester, Mass., is contained in a considerable quantity 
of menaccanite and magnetite. A hole was made through the centre of 
the specimen almost immediately, leaving the corundum projecting, but 
all of its surface was acted upon, though at a different rate from that of 
the magnetite. The iron minerals were so much softer than the corun" 
dum, that they worked out before the corundum had an opportunity of 
being very much abraded. A conical hole was made in the topaz pebble, 
which in a few seconds longer would have penetrated it, and the pebble 
would have broken in two from the increase of the hole towards the 
sides. The corundum crystal had also a coilical hole made in it, large 
enough to put the little finger through. The face of the black diamond 
exposed to the blast was originally rough, but became quite smooth at 
the end of the experiment. A microscopic examination of the surface 
of each of the specimens showed a surface, ground exactly like the rolled 
surface of stones exposed to abrasion against harder substances in 
water. There was nothing peculiar about it, except that wherever 
there was a difference in the composition, there was a difference in the 
depth of the pitting which is so characteristic of abraded surfaces. 

In order to ascertain what the direct application of these facts to 
buildings might be, I commenced a series of examinations on the com¬ 
position of the dust of cities. I found it made up of organic material 
of various kinds and sizes, of very minute, but very sharp particles of 
iron, mixed occasionally with other metals, and of considerable quanti¬ 
ties of clear and very sharp quartz, with some feldspar, together 
with a few other minerals, such as are found in the paving stones and 
dirt of cities, ground to an impalpable powder. After a heavy rain, 
preceded by high winds, I was soon able to collect considerable 
quantities of sharp quartz sand, which could be 1 easily studied with a 
microscope. On a March day we are often made aware how sharp 








11 


such sand sometimes is. When such dust is carried by high winds of 
a velocity of from 30 to 90 miles an hour, it abrades the surface of all 
the soft and, after a time, of some of the harder stones, which cannot 
resist the power of this dust, hurled against it at such high velocities. 
If the surfaces of some abraded stones are examined with a microscope, 
grains of a material of a nature quite foreign to those of the stone will be 
found adhering to it. This effect is much more prominent in cities than in 
the open country, as it is there aided by the city gases, but it takes 
place in the country as well. I have not unfrequently seen buildings in 
the open country whose abraded surfaces were due to this cause alone. 

These experiments prove the fact beyond doubt that even the hardest 
substance may be abraded by a comparatively soft material hurled against 
it at a high velocity. I had intended, at the close of these experiments, had 
I been able to get the use of the sand-blast machine, to gradually reduce the 
hardness of the sand and increase the velocity of the blast until I should 
have used flour, so as in this way to study the action of the soft material 
contained in the city dust, upon the stone. I am satisfied, from many hun¬ 
dreds of observations made on monuments and on the surfaces of build¬ 
ings, both in this country and abroad, that some at least of the decom¬ 
position that is attributed to other causes is due, to the action of the 
winds. There are many places where the falling out of the mortar be¬ 
tween the stones, and the rounding of the corners at that particular point, 
can hardly be attributable to any other cause. Usually such stones as 
these, unless they are very soft, have the abraded surface quite as hard 
as the rest of the stone, while those which are disintegrated from the 
loss of any part of the constituents of the stone, whether they are super¬ 
ficial or produced from internal causes, are always easily rubbed into 
sand. There are many instances of this character in the sandstones 
used in some of the old buildings of Europe, the binding material of the 
grains of which has been entirely silicious, the stones being worn and 
rounded in this way so as to produce a very curious effect upon the 
weather-beaten surfaces. Independent of the effect of the sand hurled 
by high winds, it must be taken into account, also, that during violent 
rains, when the wind is high, the pressure of the wind will force into the 
stone several percentages more moisture than it would under other cir¬ 
cumstances. If buildings so exposed to the action of rain blown against 
it by the wind have the usual architectural defects of flat surfaces and 
mouldings not undercut, the disintegration of the stone composing 


12 


them must be very rapid in cold climates. If, in addition to this action 
of the wind, any part of the binding material is dissolved out by the 
rain, the decay of the stone will be very rapid, and will be produced 
usually without any symptom of flaking, because the surface will be 
worn away before the decomposition has gone far enough into the in¬ 
terior to cause the flaking to take place. The decay, therefore, will 
generally remain unnoticed. 

In the selection of stones for building purposes, too little attention is 
given to their microscopical characters, and sometimes, when they are so 
examined, too much stress is laid on phenomena of little importance. 
It will not do to say that because a rock contains a mineral that has already 
commenced to decompose, as shown by the examination under the micro¬ 
scope, therefore this stone is valueless. I have, in my collection of micro¬ 
scopic slides, several sandstones, the feldspar in which has commenced 
to kaolinize, but in which the decay has been arrested. This decompo¬ 
sition undoubtedly took place in these sandstones, which are of triassic 
origin, previous to the degradation of the rocks which now compose 
them. When these were ground up, and their elements redistributed 
to form the sandstone, there seems to have been a cessation of the causes 
which produced the decomposition, which was arrested, and has not 
since, so far as we can see, advanced any further in the rock. 

The characters which it is important to observe, are whether there are 
contained in the stone, minerals which are either already decomposed, or 
are likely to become so; whether these minerals contain water in cavities 
in considerable quantities, or whether, either by disintegration or by 
the looseness of the binding material, the stone contains so many inter¬ 
stices or fissures that it is likely to absorb large amounts of water, which 
may either attack certain of the constituents, causing them to swell, or 
may itself, under the influence of a severe climate, have sufficient power, 
in the form of ice, to disintegrate the stone. The examination of the 
stone in the quarry shonld'be conducted as a whole, and not with refer¬ 
ence to a particular part of it, for it not unfrequently happens that 
stones composed of exactly the same minerals have entirely different 
properties, as granite and gneiss, for example, and yet one of them may 
not be a proper stone for outside construction. The age of the stone, 
since its extraction from the quarry, may or may not be in its favor. 
Nearly all stones are weaker immediately after their extraction 
while they hold the quarry water, than after they have lost it. 


13 


Most stones after long exposure, more especially if they have not been 
uniformly moist, absorb more water than when they are fresh, and are 
therefore more likely to disintegrate from frost than when they were 
younger, or than if they were kept uniformly moist. Certain rocks ex¬ 
posed to high heat or to severe cold, lose their power of resistance along 
irregular lines of weakness, and tend to disintegrate, and this effect may 
be produced by artificial heating as well as by climate. Stones, there¬ 
fore, which endure exceedingly well in one climate may not stand in 
another, as witness the attempt to use in this city certain limestones 
and sandstones which had stood exceedingly well abroad. The particu¬ 
lar place where the structure is to be erected, whether in the city or 
country, is to be considered. In the city there are noxious and corrod¬ 
ing gases, coming either from fuels or manufactories; the dryness or 
dampness of the ground is to be considered, and whether the particular 
spot chosen is well ventilated or not; in the country, whether the air is 
humid or dry, or whether there are prevailing high winds carrying sand. 
All these, and many other circumstances, have great influence on the 
durability of building-stones, and should be carefully considered before 
expensive structures are undertaken. 

Building-stones may be divided into three general classes: first, the 
different varieties of granite and granitic rocks; second, the marbles, 
which may have a coarse or granular structure, and may be either lime¬ 
stone or dolomite or serpentine; third, the sandstones, which may be 
composed of material having an organic, an argillaceous, a ferruginous, 
a calcareous or a siliceous binding material. Slates are occasionally used 
in building, but not frequently. They are subject to peculiar forms of 
decomposition when they are used as roofing material, about which little 
need be said, because the decomposition which they would undergo in 
such very thin sheets would hardly take place when they are used in 
thick pieces in the construction of an ordinary building. Besides 
these stones, there are a few others which are sometimes used 
in the vicinity where they are found, such as various kinds of trap or 
basalt and serpentines; also steatites, and some other very soft rocks. 
Their use, however, is not common. Each of these stones is subject to 
its own particular kind of decay, which may be either chemical or me¬ 
chanical. 

Granite is made up of quartz, feldspar and mica, and is considered 
entirely impervious to moisture, and until the two great fires of Chicago 


> 


14 


and Boston, was believed to be an almost indestructible rock. Recent 
investigations have shown that it contains within itself many elements 
of destruction. Of the minerals which compose it, quartz is the only 
one which is not liable to be represented by several species. While 
orthoclase is generally the principal feldspar of this rock, it may be 
replaced in part or in whole by microcline, oligoclase, labradorite or 
albite. The mica may be muscovite or biotite, and possibly other vari¬ 
eties. The accessory minerals which either accom£>any it, or in some 
cases reidace almost altogether some one of the regular constituents, are 
amphibole, pyroxene, epidote, tourmaline, and in certain varieties of 
Swedish granites, achmite. Of the other minerals, it is estimated that 
nearly two-tliirds of all the known mineral species are found in sienitic 
rocks. At least ten of these accompany, in more or less large quantities, 
most granites. These are garnet, titanite, zircon, apatite, magnetite, 
menaccanite, hematite, pyrite, pyrrhotite and rutile. The variety of 
the species of feldspar and mica, as well as the way they are put 
together in the stone, changes both its appearance and its physical 
qualities. As the quartz is the hardest of the minerals, it might be 
supposed that it would give its hardness to the stone. This mineral, 
as it occurs in the interstices between the feldspar, seems to have been 
formed last. It is everywhere more or less granular and brittle, so that 
while it is harder than the feldspars, it does not make the rock hard. 
Crystals which are themselves quite friable, may be separated from it by 
slight taps of a pointed tool. There is in the School of Mines’ collection 
a beautiful mass of epidote crystals two or three inches long, from New 
Hampshire, which when it arrived was entirely embedded in quartz, 
and was separated from it in this way. The feldspars are the minerals 
which give both the character and the hardness to the rock. This is due 
almost entirely to the condition of the feldspars, and although the quantity 
contained in two specimens may be the same, the character of the rock 
may be entirely different. When the crystals are distinct, and present 
highly polished cleavage faces, the rock will be hard; when on the con¬ 
trary, they are lamellar and loosely aggregated, the rook will be soft. 
Not only are the granites of different hardness, but they possess different 
rates of expansion under heat, which, as will be shown, is one of the 
principal reasons for their disintegration. The power of absorbing water 
is also quite different. The amount of water likely to be absorbed by 
a well dried granite is a little less than one per cent., but when it has 


15 


been exposed a very long time to a hot, dry climate, and has become 
slightly disintegrated, it will, as shown by experiments made on the Lon¬ 
don obelisk, absorb several times that amount. This water, and the differ¬ 
ent rates of expansion of its constituent minerals, is the reason why 
granite spalls in cases of buildings exposed during large conflagrations. 
Granites are also of different degrees of fusibility, depending on the 
quantity and the kind of feldspar contained in them; those containing 
albite are much more readily fusible than those containing orthoclase, 
all the more so, if the albite is, as it often is, in thin lamellse. The 
amount of mica present will also influence both the texture and the 
durability of the stone. If it is scattered through the rock, about 
evenly diffused, and not in too large quantities, the rock will be strong, 
if the feldspar crystals are of the proper character. If it is in bunches 
it will render the stone weak where the bunches occur. If distributed 
in directions which are parallel to each other, it may make the rock so 
weak as to give it a tendency to cleave in the direction of the mica 
planes. 

The presence of hornblende minerals does not seem to affect the 
strength of granite rocks, except so far as after very long periods it is 
liable to certain kinds of decomposition. It does not generally occur 
in planes arranged in given directions, and is rather in long crystals of 
a more or less fibrous nature scattered through the rock. Its fracture 
is somewhat fibrous, so that it seems rather to tend to consolidate than 
to weaken the stone. The granites containing hornblende are amongst 
those which have longest resisted decay and disintegration. It replaces 
the mica forming the famous syenites which were so much esteemed as 
building materials by the ancients. When hornblende is replaced by 
pyroxene, the rock is not durable. Pyroxene is much more brittle and 
breaks with a much more granular fracture than the hornblende min¬ 
erals. It does not tend to form fibrous masses, so that the rock is much 
more brittle than those containing the hornblende minerals. Some New 
England granites are liable to chip and break, owing to the presence 
of this mineral, which, whenever it is bunched, is liable to crack out in 
nodules, or when it is in considerable quantity, evenly distributed, to 
make the stone brittle as a whole. These two minerals exist together 
in certain granites which are then all the stronger as they contain less 
pyroxene. When the rock contains hornblende alone, it is usually very 
tough. The feldspar present in such cases is almost invariably ortho- 


clase, which, as it is the soundest and most permanent of the feldspar 
family, makes the stone a very desirable one. 

It is very generally believed that granite cannot decompose. The 
kaolinization of the feldspar goes on with exceeding slowness, and ex¬ 
cept under conditions very favorable to it, would not be likely to have 
much influence for many years. Not so, however, with the other min¬ 
erals which compose it. The quartz is often full of microscopic bub¬ 
bles, carrying liquids liable to freeze by cold or to be transformed into 
gas by heat, so as to produce a maximum of tension. The least space 
between the minerals would thus tend to become widened by the lapse 
of time. When the mica is biotite, the rock is more liable to decay on 
account of the ease with which this mineral decomposes. When it is 
present in large quantities, it makes the rock tender from the readiness 
with which it cleaves. When it is stratified it makes parallel lines of 
weakness along which the stone splits. The quartz contained in the 
rock is usually full of cavities, some of which are microscopic and 
others macroscopic, which may disintegrate the quartz, either by the 
expansion of the liquid by heat or cold, and thus render the rock it¬ 
self more liable to absorb moisture than before. Independently of all 
this is the general change in the structure of all granites when exposed 
to very great variations of temperature, which cause minute fissures 
along the lines of least resistance, which are constantly increased in 
width and depth, causing the stone to become weakened and finally to 
disintegrate. This is most frequently seen in granites in which the tri¬ 
clinic feldspars occur, and is most prominent in them when the crystals 
are large. The rate of expansion and contraction being different in three 
directions, when the variations of temperature are very great, cause sep¬ 
arations .to take place along the lines of cleavage of the crystals, which 
is the line of least resistance, which very soon produces irregular lines 
of weakness. These are rarely apparent to the eye, and may have been 
developed to a considerable extent without the possibility of detection, 
except by a microscopic examination, which is very rarely made. Ex¬ 
amples of this are to be seen in the obelisk of Luxor, the one on the 
Thames embankment, and the one in Central Park, which are disinte¬ 
grating rapidly, and in a few score of years, if left exposed, will prob¬ 
ably be beyond remedy. 

The decay which takes place in the granites is either chemical or 
mechanical. If chemical, it is either the result of the very slow decom- 


17 


position of the feldspars and micas of which they are composed, or else of 
some mineral contained in the granite which decomposes easily, and by 
its swelling either causes the stone to flake, or by its decomposition to be¬ 
come porous and leave the stone free to be mechanically acted upon by 
frost and rain. If mechanical, it is the result of the weakening produced 
along the lines of least resistance by the continued expansion and con¬ 
traction of the stone where it is exposed either to very great, but grad¬ 
ual changes of temperature, or to sudden ones often repeated. This 
causes the stone to disintegrate, and is a simple mechanical action with¬ 
out any chemical change. Such weakness has been developed in some 
granites from this cause, as to produce considerable chipping of the 
stone in the quarry. It frequently occurs in the granites of New Eng¬ 
land, to such an extent as to almost convert the exterior of the rock to 
sand, or to break it up into very small pieces, so that it can be easily 
removed without blasting. If, in addition to this cause of mechanical 
weakness, the rock should contain pyroxene, it would be safer to reject 
the stone. The power of the action of frost is much greater than is 
usually supposed. It takes a long time for frost to enter any 
considerable distance into the interior of a stone. When it has once 
entered, it takes a much longer time to thaw out. I have known of cases 
where granitic rocks which were quite warm on the outside, where they 
were exposed to the sun, froze solid, liquid cartridges introduced 
into the drill-hole to blast the rock. In this case there was the 
rock in front, which had expanded by the heat at one rate, the rock 
behind, which was expanded by the frost at a different rate, both 
forces acting at the same time, both tending to weaken the stone. 
Sdch effects do not act to any extent when the stone is in con¬ 
structions, but it may have acted previous to its being placed there, 
and have seriously weakened it. Such disintegration takes a very long 
time; has not been observed to any great extent, so far as my knowledge 
goes, in the building materials of this country, except in the quarries; 
and is easily provided against by a careful selection of the stone. It is, 
however, very common in some of the out*crops of quarries, especially 
in those granitic rocks which contain a very large proportion of mica, 
which mineral, as it is very easily cleaved, is always a source of weak¬ 
ness ; especially so when it occurs arranged in parallel planes, or 
contains substances likely to undergo a chemical change. In many 
instances granites have b’een discarded by our architects as building 


18 


material because they do not resist fire. It is, however, not fair to 
judge of the quality of a granite by its more or less great fusibility. 
Granites exposed to the air always contain moisture. All of them are 
fusible, and all will spall and crack under the influence of such intense 
blow-pipe heat as occurred in the great fires of both Boston and Chi¬ 
cago. No other building-stone would have resisted those fires any better* 
for the limestones would have been burned to quick-lime, as they did, 
and the sandstones would have disintegrated in the same way. There is, 
however, great choice to be had in the selection of granites, on account 
of their mineralogical constitution, and the composition of the minerals 
which they contain. The amount of decay of which they are suscepti¬ 
ble is the least of all natural stones, except some of the sandstones 
which have a siliceous cement. Granite to be used as a building-stone 
should be of uniform grain, free from dark spots or aggregations of 
minerals in bunches. It is all the better where there are the fewest num¬ 
ber of minerals, especially of the triclinic feldspars or larger mica plates, 
or pyroxene contained in it. It should especially be free from iron com¬ 
pounds, which are likely to oxidize. The decompositions of any kind, 
whether chemical, physical or mechanical, are very slow, and take place 
for the most part only in stones which a careful examination "would have 
caused to be rejected from the outset. A good granite will last for ages, 
but no granite with very large or very irregular sized feldspar crystals, 
or minerals likely to become oxidized, will last. It may even be said of 
bricks that when they are improperly burned, or made of a poor qual¬ 
ity of clay, they will exfoliate, fall to powder or even be dissolved, 
while some of the Roman brickwork has stood for two thousand years, 
and is still in good condition. 

Gneiss is formed of the same constituents as granite, and is subject 
to the same causes of disintegration, only in a much higher degree, as 
the more or less of lamination, which is due to the arrangement of the 
mica, causes it to split easily. There is every possible gradation of rock, 
from a recognizable granite through gneiss to a mica slate, depending 
on the relative abundance of the different minerals. Owing to the 
presence of so large an amount of mica, and that it frequently in 
addition contains a considerable amount of pyrites and other sulphides, 
it is likely to be a perishable stone. This is seen frequently in the 
rock of New York Island and elsewhere, where the rock is so soft on 
the surface that it is frequently possible to remove it to a depth of 


19 


several feet with a pick and shovel. Generally, when hard stone is 
reached after the removal of such a surface, the rock is worthless for 
several feet below, and will go on disintegrating if put into foundations 
or structures of any kind. I have seen slabs of gneiss 4 feet long by a 
foot thick, in retaining walls, so thoroughly disintegrated by the de¬ 
composition of the pyrites it contained, that it could be picked to 
pieces with the nail. These kinds of decomposition are constantly seen 
in the repairs to the foundations of old houses and sometimes in struc¬ 
tures, but generally as such stone is buried it attracts but little atten¬ 
tion. In retaining walls, whole sections built in the upper part of this 
city have become so weak in less than 20 years, that they have had to be 
rebuilt. It is also liable to exfoliation when set in the structure in* 
such a way that the mica planes are subject to the action of heat. 
Occasionally, when the plane of the stone is at right angles to the 
quarry-bed, the quartz, or feldspar, when embedded in mica so that it is 
on all sides of it, will chip out. Generally, when properly used, if the 
stone has been carefully selected, it is a durable building material. 

It is not generally known that rocks which are hard, and subject to 
almost no decomposition in one climate, may be thoroughly decomposed 
to great depth in another. Such a decomposition seems to be going on 
in the gneiss of New York Island. The water taken from the artesian 
wells of this city, and of the vicinity, often contain from 12 to 20 grains to 
the gallon of mineral matter in solution, and some of these, which have 
been sunk to the depth of 700 feet and over, contain from 6 to 13 grains 
to the gallon, of carbonate of soda, showing that the rock is undergoing 
decomposition at great depth as well as at the surface. It has generally 
been supposed that the brackishness of our artesian-well water was 
owing to the fact that the dip of the strata is such as to bring towards 
the center of the island, from both sides, the salt water of both of the 
rivers, but this could not explain, even if it was true, for such depths 
the presence of such large quantities of carbonate of soda. I have seen 
gneiss in North Carolina so thoroughly decomposed over a large 
area, and to a great depth, that it was soft like clay. In the vicinity 
of Van Tyne’s Station, on the Richmond and Danville Railroad, there 
is an outcrop of a gneiss rock so decomposed that, while preserving to 
the eye alone all the characteristics of an unaltered rock, it can be made 
into balls like clay. This decomposition has been traced to a depth of 
250 feet. In the same neighborhood is a trap-dike, over 60 feet wide, so 


20 


thoroughly decomposed that it can be shoveled up like dirt, with no indica¬ 
tion that it ever was a rock, except that here and there, there are boulders 
10 or 12 inches in diameter with an outside crust like the dirt, but a little 
harder, but having the unaltered rock in the center. I traced this dike 
for half a mile, and found it everywhere in the same condition. The 
railroad cuts through it in one place where the sides of the cut are over 
50 feet high, showing the rock thoroughly decomposed from top to 
bottom. Such decomposition is quite common in modern volcanic 
rocks, and the soils which they furnish are much sought for by the wine 
growers. 

Mica slate is composed of quartz and mica. It is of necessity lami¬ 
nated, and contains a large number of minerals included within it. It is 
fusible, and not very suitable for structures above ground, nor hardly 
for those below. The mica is generally biotite. It frequently is so 
rotten, from the action of decomposing pyrites, and from the decomposi¬ 
tion of the mica, that it is unfit for anything but gravel. Such material, 
when thoroughly rotted, makes good roads and hard walks. It is 
sometimes used in buildings, but not often above ground. Its decay is 
not very rapid, but is quite sure in time, if the mica predominates, as it 
usually does. 

Under the name of trap, several varieties of stone are included. As 
they cannot usually be got in large pieces, on account of their brittle¬ 
ness, they are used only occasionally for buildings, when they do not 
have to be carried any very great distance. They are composed mostly 
of a triclinic feldspar, which is often labradorite with pyroxene, peridot, 
magnetite, menaccanite, sometimes apatite, and a mica which is gener¬ 
ally biotite. They are almost always accompanied by some form of 
chlorite, which is usually a product of decomposition. They are often full 
of small cavities which are lined with chalcedony and the zeolitic minerals. 
The decomposition seems to be caused by the solution of certain parts 
of the rock and a greater or less precipitation of the elements dissolved 
in the stone itself. The rock is usually tender and brittle while the 
quarry water is in it, and quite tough afterwards. Its decomposition is 
exceedingly slow. No great buildings have been made of it, on account 
of the impossibility of getting it in large pieces. 

Serpentine is a hydrated silicate of magnesia of variable composition, 
and associated with a number of minerals of the same general composi¬ 
tion. It is also a product of decomposition of other minerals. It is a 


tough but soft stone, and may be in all conditions from roughly lam¬ 
ellar to almost granular. It is associated with chromite and magnetite, 
and frequently contains considerable quantities of calcite, and with cer¬ 
tain fibrous minerals, which are sometimes a variety of the rock itself, 
and sometimes fibrous varieties of the amphibole series. When it is 
contained in calcite it is often used as an ornamental stone. Not unfre- 
quently it is associated with the variety of steatite known as soapstone, 
when it becomes a serious impediment to the use of the steatite. Both 
stones have been used for building, although they are so soft as to be 
unsuitable for large structures. They have stood in some places where 
they have been used for 150 years; in others they have commenced to 
decompose in a much shorter time. This is owing to the variable 
nature of the rock, which is rarely ever homogeneous, either in com¬ 
position or structure. When they are quite pure, both steatite and 
serpentine, though so very soft, are practically indestructible rocks. 
They are not generally, however, suited for use in large cities, on 
account of the considerable quantities of carbonates they contain, which 
are easily acted upon, and cause the stone both to lighten in color and 
to disintegrate on account of them. They can generally only be had 
in small pieces, which is objectionable in a building stone. 

Under the name of porphyry are included a very large number of 
stones which have a compact, generally dark-colored base, with crystals 
either of feldspar or of quartz of a different color showing in them. 
These are generally either quartz or some kind of feldspar, or both. They 
usually occur in dikes. The stone is very hard and tough, and generally 
can only be had in small pieces. It has many different varieties of color, 
both of base and crystal, and is a very beautiful stone. It was very 
much sought for by the ancients, and was extensively used, both for 
construction and ornament. None of these stones used in their buildings 
show any trace of decomposition, and those found in the ancient ruins 
show only a slight diminution of the polish, which, where it was made 
very high, seems almost as indestructible as the stone itself. In places, 
however, it is quite frequently decomposed. I have seen considerable 
quantities of it decomposed to a clay, looking exactly like the unaltered 
rock, even to the colors of the crystals, with every ingredient, except 
the quartz, turned into a plastic clay, so soft that it could be easily 
moulded in the hand. This decomposition has, however, so far as I 
know, only been found in situ. The same kind of decomposition takes 


22 


place in the conglomerate rocks of Lake Superior, which are porpliry tic, 
when all the constituents of the rock, except the quartz, are sometimes 
transformed into a soft clay. The entire replacement of the base of the 
rock by native copper is quite common, both in the conglomerate and 
amygdaloid rocks of that region. 

Slates are little used in construction, both from the difficulty of get¬ 
ting out large blocks, on account of the ease with which they cleave, and 
also because of the unattractive color which most of them possess. 
They are sometimes used for building in the vicinity of large quarries, 
only those pieces, however, being used which do not readily cleave. They 
must always be placed in their quarry-bed. When placed in a vertical posi¬ 
tion they are very apt to laminate, but in the quarry-bed they have stood 
for hundreds of years. Some varieties are subject to a superficial decom¬ 
position, which changes the color to an unpleasant yellowish-gray, but 
it is very slow. When the slate contains pyrites its use must be avoided. 
Slates cannot be carried far as they are not much esteemed, but they are 
a durable stone when of good quality, and only objectionable on account 
of their color. They are used chiefly for roofing purposes, but are being 
gradually superseded of late years on account of their weight, it being 
more expensive to build a roof strong enough to support slate than one 
for a lighter roofing material. In the older parts of the city, where the 
Dutch traditions were still in force, the roofs were almost invariably 
made of slate and tile, and placed at an angle of 45 degrees, so as to 
prevent the accumulation of snow upon them. Tile having been found 
altogether unsuited to this climate, was abandoned entirely; but slate 
still continued popular until tin and other materials began to supersede 
it, as they are now doing to a great extent. It is not ordinarily subject 
to decomposition, and when this does take place in it, it is usually attrib¬ 
uted to the presence of pyrites. I have, however, within a year, seen, 
on the roof of a house in Massachusetts, rather thick roofing slates, 
entirely free from pyrites, which were placed there twenty years ago, so 
decomposed that they will not bear the least pressure, and are broken 
into small pieces by the force of the wind. This decay is accompanied 
by a change of color; originally blnish-black, it changes almost to 
brown. After a little while, such slates will not only be no protection to 
the roof, but their j>resence will be an absolute detriment. This form of 
decay, however, is not found in slates of good quality, and is more of a 
lithological curiosity than a real danger to be guarded against. 


23 


Slate was formerly much used for ornamental purposes, and where 
that of good quality was selected it answered perfectly well, as it is cap¬ 
able of receiving very high polish. Owing, however, to the difficulty of 
securing a slate which was compact and hard, this use of them seems to 
have been abandoned. They are, however, still extensively used to make 
enameled surfaces for interior decoration. The old graveyards of New 
England contain many tombstones of slate dating back nearly to the 
close of the seventeenth century. Most of the slate headstones in 
Trinity churchyard are to all appearances entirely unaffected, and seem 
to be as strong and perfect as on the day they were placed there. 
Two of these were erected in 1691 and 1692, on the north side of 
the church, and were cut on both sides. One of them, that of 1691, 
shows no sign of deterioration, except an occasional spalling of very small 
pieces ; the inscription is quite sharp. The other is very much broken on 
the lines of pseudo-cleavages, produced by pressure; but the surfaces 
of fracture, though produced many years ago, are quite fresh. The faces 
of both these stones are somewhat gray from alteration. 

Of the different varieties of marble, those which are granular are the 
ones which are generally most easily attacked, and of the limestones 
and dolomites, the former seem to be most readily acted on, while those 
marbles which are composed of a mixture of limestone and dolomite, 
are very easily affected by the weather, the limestone going first and 
leaving the dolomite. Most of this weathering takes place on buildings 
in the country, or standing by themselves, by the decomposition of the 
minerals contained in the interior, which can easily be prevented by prop¬ 
er selection of the stone. Where the buildings, however, are in a con¬ 
fined space, especially in large cities, the action of the city gases is such 
as to attack the limestone, leaving the less easily affected dolomite alto¬ 
gether uninfluenced at first, and afterwards, according as there is more 
or less of lime present in it, causing it to crumble, or leaving a very 
rough surface. There are a number of notable examples of this decom¬ 
position in some of what were once the most beautiful tombs in our city 
churchyards, which have literally fallen into sand by the solution of the 
lime by the city gases. The commencement of this action may be seen 
on the monument to Alexander Hamilton in Trinity churchyard, and on 
the Emmet monument in St. Paul’s. The same action is frequently 
seen in the dolomite quarries, as at Lee, Mass., where the limestone is 
dissolved out, leaving the dolomite as a sand with crystals of tremolite 
lying loosely in it. 


24 


The churchyards of the country also are full of stones undergoing 
this kind of decay; which, however, progresses much more slowly there 
than in the city. One of the most remarkable instances of decay of 
stone can be seen in the southern division of Trinity churchyard, near 
Broadway, about half way between the church and Rector streets, in the 
shape of a square marble monument with an inclined top now about 
four feet high. Originally this was twelve feet high, supported on 
polished columns, with an open space underneath, in the center of 
which was an urn. This monument was erected in the year 1820 to 
the memory of Grace Lyde. It was made of highly polished marble, 
and when it was put up, was one of the most beautiful and graceful 
monuments ever erected in any of our city churchyards. In the year 
1860, the name on the monument had already become illegible. 
About five years ago it was in danger of falling, and an order was given 
by the church authorities to repair it. It was found, however, that the 
stone was so badly decomposed that it would not hold together under 
the pressure of its own weight, so that the monument could not be re¬ 
paired. It had to be taken down. All of it that could be used was put 
up in its present shape. The decay has still further progressed since 
then. The stone is so soft that it easily crumbles between the fingers^ 
and it is rapidly falling into sand. The inscription has entirely disap¬ 
peared, and in a few years longer nothing will be left of it but sand. 
The Capitol, at Frankfort, Ky., was built in 1837 of a very fine-grained 
limestone, with some excess of carbonate of magnesia. This has become 
acted on, so that the building is completely coated with what appears to 
be a very fine wash, which is very adherent, but can be easily scraped 
off with a knife. It consists of carbonate of lime. The interior of the 
stone has suffered no decay. 

As a general rule when a limestone contains much pyrites it should 
be discarded, but it does not always follow as a necessary consequence 
that the presence of pyrites in stone is of necessity a disadvantage. As 
a general rule the presence of marcasite is. Of the ordinary pyrites 
some varieties do not decompose, while others do; the presence of such 
varieties as decompose may disfigure the stone, if in small quantities only, 
or may cause it to swell and disintegrate if in large quantities. In com¬ 
pact stones its presence has but little influence; in porous ones it is gen¬ 
erally objectionable. But no absolute rule should be laid down, for, 
while as a general thing it is to be avoided, it may be harmless. In gen- 


eral it may be said that the presence of much pyrites makes the stone 
unfit for use in the exterior of buildings. It is often a matter of sur¬ 
prise that some limestones do not stand here, either in houses or in 
graveyard monuments, for any great length of time, while they have 
stood for thousands of years both in Italy and in Greece. This is, how¬ 
ever, very easily explained, from the fact that in these countries in the 
situations where they have stood, the air is comparatively dry, and also 
does not contain so much, if any, of the injurious gases which affect our 
stones. In looking over our churchyards —Trinity, St. Paul’s, St. John’s, 
and Trinity cemetery—we find that the effect of weathering upon the 
marble is very curiously developed. If the stone is placed vertically, 
according as it is more or less compact, or is composed entirely 
of carbonate of lime, or partly of lime and dolomite, the polish on the 
stone will be very much deteriorated in the course of ten or fifteen 
years, and may, on certain stones, sometimes be entirely gone in less 
than that time. 

There are many stones in Trinity and St. Paul’s churchyards, placed 
there within the century, where the inscriptions have been entirely 
obliterated, leaving in their place a very rough surface, which scarcely 
shows any trace of the stone ever having been worked. In Trinity 
cemetery, at One Hundred and Fifty-sixth street and North River, 
where the air is much purer than it is in the city, some of the marble 
headstones have become so rotten after twenty years’ exposure, that they 
will not bear their own weight, and crumble from the least friction, even 
of the hand. As a general rule the finer the texture and the closer the 
crystals are arranged in the stone, the less this disintegration has taken 
place. The monument erected in 1814 to the Sieur Rochefontaine, who 
commanded the French forces during the Revolution, was so far decom¬ 
posed, that in the year 1885 the inscription was recut by order of the 
Corporation of Trinity Church. The monument not far from it, erected 
to G. F. Cook, the famous actor, by Edward Kean, in 1821, was repaired 
by Charles Kean in 1846, and again by E. A. Sotliern in 1874. Except 
on the south side, the inscriptions are now very much defaced. It was 
necessary to recut the inscription on the monument to Alexander Hamil¬ 
ton in Trinity churchyard, in 1885. The monument to Alexander Brad¬ 
ford, New York’s first printer, erected in the year 1752, was so far de¬ 
faced and crumbled, that in the year 1868 the Corporation put up & fac¬ 
simile of the original stone. This is so far acted on already that sand 


26 


can be brushed off of its surface with the hand. The inscription on the 
altar tomb near that of Alexander Hamilton, in Trinity churchyard, is 
so far defaced, from the solution of the stone, that only a slight depres¬ 
sion, where the lettering once was, can be distinguished on the surface 
of the stone. As the result of the examination of the New York 
City cemeteries, I am confirmed in the opinion that, in general, 
limestone, whatever its character, is entirely unsuited in this climate 
for the construction, in cities, of monuments which are to be ex¬ 
posed to the air. One of the most remarkable destructions of this 
kind which has ever come within my observation is that of the cathedral 
at Douai, in France; the whole of the outside coating of Caen 
stone of this church is gone, leaving nothing but the filling of 
brick and rubble to support the interior of the church. It is said 
that the whole of the outside of Westminster Abbey has already been 
replaced twice, and will soon have to be replaced a third time. The 
exterior of Cologne Cathedral was hardly finished before they commenced 
to repair it. Notre Dame, and the Tour St. Jaques, in Paris, require 
constant attention and repair. 

The rate of disintegration in such stones used for buildings can be 
very much lessened by filling the pores of the stone where decay has 
already commenced, or previous to its being put into the building, with 
some substance like oil, paraffine, or sulphur. In the case of the 
experiments that have been tried upon the Houses of Parliament, it has 
been found that sulphur in solution has answered better than any of 
the other substances used as a remedy against this decomposition. It 
has also been found that the filling of the pores of that stone is gen¬ 
erally useless if it has been done on the outside only. I saw several 
cases where the pores of the surface only of the stones, having been 
filled with sulphur, had retained their surfaces during twenty years of 
exposure, and were only just beginning to exfoliate. Where the remedy 
has been applied to the six sides of the stone previous to its being put 
into the structure, it seems to have been effective, such stones, after 
ten or fifteen years of exposure, showing no signs of decay. 

Almost everything has been tried on the Houses of Parliament, but 
nothing as yet has been found to be successful. This is owing partly to 
the fact that a large proportion of the stone was unfit to put into any 
building under any conditions. The Commission which selected the 
quarry from which the stone should be taken was discharged, when their 


27 


report was made, and no one was responsible for the stone selected, after 
this quarry was found to contain an inadequate supply; and when it 
failed, stone was obtained almost anywhere that it could be had. The 
destruction has been so rapid, and the decomposition of their highly 
ornamented surfaces has extended so far, that in the case of finials and 
the small pinnacles which they surmount, they are now replaced by cast- 
iron painted the color of the stone. 

From want of care in its selection, I have seen the calcaire grassiere of 
Paris decompose so rapidly, that the stones had to be removed after a 
few years. Up to 1860 there were on the side of the Louvre next the 
Seine, between the passages that enter the Place du Carousel next the 
Tuileries, large stones which had fallen entirely into sand to a depth of 
over 6 inches, and the remaining portion was so soft that it could be 
picked to pieces with the finger-nail. This same decomposition took 
place also in certain sandstones with a calcareous binding material, 
which was much used in Paris at this time. I have often gathered 
fossils in the sands of the quarries from which both these kinds of stone 
were taken, certain beds of them on the outskirts of Paris, being the 
favorite resort of fossil hunters. In this case there was both the 
original want of consolidation of the stone and subsequent decay to 
make it weak. 

The silicates of soda and potash with which the surface of the 
Louvre in Paris was washed in 1858, seems to have been quite effectual. 
In this case a silicate of lime was formed, which has protected the 
surface from further action of the weather, but on the Houses of 

t 

Parliament, owing to the bad quality and the extremely soft and porous 
nature of the stone, which required that the pores should be filled at 
the same time that the surface was indurated, it does not appear to 
have been successful. 

Some years ago attempts were made to introduce Caen stone into 
this country, as it had proved so satisfactory in many buildings in 
England and France. A number of houses of this stone were erected 
both in New York and Brooklyn. About the same time the interior of 
Trinity Chapel on Twenty-fifth and Twenty-sixth streets was lined with 
this stone. In less than ten years the Caen stone used on the outside of 
buildings in Tenth street, between Fifth and Sixth avenues, began to 
exfoliate, and the fine carving crumbled to pieces before anything could 
be done to preserve it. The rest of the front of some of these buildings 


28 


has been preserved till this time, only by keeping it constantly and 
carefully painted. To be effectual, this paint must be renewed every 
four or five years. The interior of Trinity Chapel, however, shows no 
trace of decomposition of any kind. The stone is a little clouded with 
dirt, but otherwise is apparently as sound as the day it was erected, 
showing that Caen stone is perfectly suitable for interior decoration 
when kept dry. Soft stones of this character can only be used in 
outside work in large cities by being kept constantly coated with 
paint. 

It is not an uncommon thing where the dolomite and limestone are 
mixed together, in slabs that have been highly polished, to have the 
limestone filling between the crystals eaten out by the gases when they 
are placed in confined places in cities, which leave the dolomite crys¬ 
tals projecting, so that the surface looks as if it never had been pol¬ 
ished. An excellent example of the commencement of such decay can 
be seen in the altar tombstones near the south entrance to Trinity 
Church, and in the Emmet monument in St. Paul’s churchyard. 

An extremely curious phenomenon sometimes takes place from the 
elasticity of limestones, which seems to threaten the immediate destruc¬ 
tion of the stone, but which it may resist for a very long time. It is not 
confined entirely to limestones, but occurs in sandstones as well. This 
is the bulging of the stone owing to heat applied entirely on one side. 
From the constant expansion of the surface the crystals seem to assume 
a certain degree of mobility among themselves, so that the stone bends 
outward if placed in a vertical position, or sags, if there is opportunity 
for it, when placed horizontally. This phenomenon has been noticed in 
some of the public buildings in Europe, but has not attracted much 
attention here. Having occasion many years ago to examine into the 
cause of a smoking chimney, I found the opening at the top almost 
entirely closed by the sagging of the cap, which was a piece of dolomite 
3 feet six inches square and 2 inches thick. I had it turned over with 
the bulged side up. In the course of another six months it had com¬ 
menced to bend in the other direction. The next year it had to be 
turned over again, and after two or three turnings it finally broke. The 
heat of the fires on the under side had produced such a separation be¬ 
tween the crystals as to cause their movement, and the successive bend* 
ings had weakened the stone so that it no longer had sufficient resistance 
to bear the strain of bending. In buildings the danger of bulging is 


29 


only that of throwing the stone out, as the expansion is on one side 
oniy, and as it can never occur except when the stone is used in thin 
slabs for facings, can always be provided against by the way in which 
the stone is fastened into the wall behind it. This same phenomena is 
often seen in the altar tombs, where the slab is not sufficiently sup¬ 
ported in the center. It is most common in the coarse granular lime¬ 
stones, or those composed of mixtures of calcite and dolomite. Some 
such tombs can be seen both in Trinity and St. Paul’s churchyards, 
which are so hollow in the center, that they hold sufficient water for the 
birds to bathe in. On most of such stones the inscription has been en¬ 
tirely obliterated. I have sometimes seen sandstones which had lami¬ 
nated in thick layers, from the solution of their binding material, curl 
away, by the heat of the sun, several inches from the stone behind 
them, in sheets over 18 inches wide, and keep bending, until they could 
no longer support their own weight. 

Certain conglomerates, which are very hard when extracted from the 
quarry, undergo a more or less rapid decomposition when exposed to 
the air. Such conglomerates as these, composed mostly of quartz or 
limestone grains cemented together by an argillaceous material, are quite 
frequent in the coal formation, and are, of course, utterly unsuited for 
building material. There are, however, other limestone conglomerates, 
notably some of those breccia which are used in the public buildings of 
Washington, where the disintegration, though much slower, is none the 
less effective. The beautiful colonnade of the Treasury Department on 
Fifteenth street, in Washington, is made of Potomac marble, which is 
such a material as this. Some years before the war, the disintegration 
of this stone had progressed so far, that the complete destruction of 
the colonnade was threatened, so that it became necessary to prevent 
its extending further. It was coated with paint, and has been carefully 
protected in this way ever since. It is not generally known that the 
front of the old Capitol building and also of the White House, which are 
limestones, commenced to disintegrate so seriously that they were 
painted some forty years ago, and have been kept painted ever since. 
The same decomposition is taking place in some of the beautiful marble 
used in the decoration of the Chapel of Durham Cathedral. 

Sandstones are generally supposed to be composed of quartz sand, 
cemented together by different kinds of binding material, such as quartz 
itself, as is the case with the Potsdam sandstone; with oxide of iron, as 


30 


in some of the sandstones of New Jersey and Connecticut; and with car¬ 
bonate of lime, with or without oxide of iron, as is the case with most of 
the stone of Trinity Church, and of the ordinary brownstone which is 
so extensively used in the construction of the fronts of our city houses. 
In some cases clay is the binding material of the sand, and in others it 
appears to be some organic compound which easily decomposes. 

When the material of which the stone is composed is fine, it is called 
a sandstone; when it is coarse, it is called a conglomerate. When the 
pieces are of the size of a hickory nut, and, as is frequently the case, of 
various colors, it is called a pudding-stone. When the pieces are angular, 
it is called a breccia. Where such siliceous rocks have undergone meta- 
morphic action, they are frequently changed to quartzites. I have often 
seen a fine-grained quartz sandstone, under the prolonged action of heat in 
a furnace, without however melting, turned into a nearly compact quartz, 
with a glassy luster, having very much the aspect of a glazed porcelain. 
A striking example of this phenomena is in the metallurgical collection of 
the School of Mines. From a simple inspection there does not seem to be 
the slightest relation between the stone before and after metamorphism. 
The stones, however, that are composed altogether of quartz, are of rarer 
occurrence than is generally supposed. The materials making up the 
rock are of variable composition, and include a great variety of minerals 
which seem to have come from previously existing rocks, which have 
been ground up and subsequently cemented together. In most cases 
the minerals contained in the stones have retained the characteristic 
qualities that they had in the original rock. The microscopic bubbles 
can still be seen moving, as they have been doing since the ages of their 
formation. The feldspars retain their characteristics, and the fact that 
the rock of which the sandstone was formed was sound, or was under¬ 
going decomposition at the time of its destruction, may still be seen in 
the sandstone. 

The structure of the stone is very different, not only from the differ¬ 
ent kinds and sizes of the grains which compose it, but also according 
to the greater or less amount of pressure exerted at the time of its forma¬ 
tion. In some, the consolidation is so loose that they are manifestly 
unfit for constructive materials; in others, there is every degree of real 
or apparent compactness. In order to fix a rule, it has been stated that 
those stones which, in a climate like our own, effervesce slightly with 
aoids, weigh less than 130 pounds to the cubic foot, and which absorb 


31 


in the course of twenty-four hours over five per cent, of water, cannot 
be good stones. Many of our sandstones, after having been exposed to 
the weather for a considerable length of time, will absorb anywhere 
from three to fifteen per cent, of water in twenty-four hours, and conse¬ 
quently stand but a short time in a building. 

When sandstone, which has a laminated structure, is not placed in its 
quarry bed, it is impossible that it should stand in the building for any 
great length of time, unless the binding material is quartzose. Sooner 
or later, from the effects of the weather, lamination will take place, and 
this may be seen in most of the sandstone pillars so common on the 
fronts of houses in former days, and on some of the ornamentation of the 
beautiful entrance to Greenwood cemetery, which is being rapidly 
destroyed in some of its upper parts by this cause, as well as decom¬ 
posing from other causes in other parts of the structure. 

Decomposition occurs most rapidly in those sandstones having 
organic or argillaceous binding materials. The former are decomposed 
by the weather, the latter very rapidly swell and disintegrate the stone, 
and very soon obliterate all traces of mouldings. Those having a fer¬ 
ruginous gangue are more suitable, unless the iron has come from the 
decomposition of pyrites, in which case the excess of sulphur renders the 
stone likely to disintegrate. In some cases the causes which have 
produced the precipitation of the iron, which binds the sand together 
have acted very unequally, so that the stone is hard in some places 
and soft in others, and consequently resists the action of pressure 
unequally in different parts. 

Most of the New Jersey sandstone has either an argillaceous or calca¬ 
reous binding material. Both are easily acted upon by the weather 
in the country and by the gases of the city, especially at points 
near the ground, or where the mouldings are improperly cut, so 
that the water is not shed from, but remains on or filters through 
them. Examples of this may be found in almost every street in large 
cities where brownstone is used. The same kind of decay may take 
place at or above the level of the ground from a different cause. If 
the superstructure has not been provided with a damp course, the 
moisture will rise in the masonry as high as 8 or 10 feet, the effect being 
the more prominent as the walls are thicker. If, in addition to this, 
the surface drainage is toward the building, instead of away from it, the 
quantity of moisture will be all the greater. Exactly the same effect is 


32 


produced when the water from the roof is not absolutely cut off from the 
vertical walls of the building. In old buildings little or nothing was 
done to prevent this action except the drainage of the foundation. 
When the foundations were properly drained, and the stone used in 
them had a siliceous cement, the bad effect was exceedingly limited in 
extent, and did not usually show itself until after a long period of years. 
When, however, the stone was a limestone, or had a calcareous binding, 
this was slowly but surely dissolved out, the effect being all the more 
rapid as the locality was more densely populated. In order to prevent 
this action, the outside of the foundation walls against the earth is 
covered with asphalt, and what is called a damp course is frequently 
made by spreading a thin layer of asphalt over the horizontal surface of 
the wall above the ground so as to prevent the water from passing 
up into the walls of the building. If there is no protection from 
dampness, as in most old buildings, and the situation is a moist one, 
there is then another difficulty which hastens the decomposition of the 
stone, which is, that on the north side the stone is uniformly moist, 
while on the south it is, from its greater exposure to the sun, dry and 
moist alternately. As the moist side is already near its point of satura¬ 
tion it sheds the water, while that which is dry absorbs and then sheds 
it, carrying some soluble material with it. 

When in sandstones the binding material is oxide of iron, there is 
frequently only just enough of it to hold the sandstone together, so that 
the least decomposition will cause the stone to disintegrate. Such 
stones as these, where there is a minimum quantity of binding material, 
are always rapidly decomposed when subjected to atmospheric influences. 

Of the sandstones having a siliceous binding material, the Potsdam 
sandstone, which has been used in the recently constructed buildings of 
Columbia College, and the siliceous triassic sandstone which was the mate¬ 
rial used in the lower part of the Cathedral at Rodez, are the best examples; 
and in these no decomposition takes place. Of these sandstones it will be 
noticed that there are two general varieties, one in which the quartz grains 
are more or less large and are rounded, but are cemented together by 
silica, a variety which may be found in any quarry from which brown- 
stone is taken, and is the only variety of that kind of sandstone which 
should ever be used for building. In the Potsdam sandstone, on the 
contrary, the grain of the quartz is quite small, its shape, when it 
can be distinguished at all with a magnifying glass, is always angular. 


33 


The stone is porous, but is cemented by silica, so that it appears, on a 
cursory examination, to be quartzite. This is the best of all building 
materials, though mouldings made of the other variety will last for many 
years, without suffering any appreciable amount of deterioration. The 
Potsdam sandstone has been but recently introduced into New York. 
It is an excellent building material. Almost the only objection to be 
made to it, is that it is difficult to obtain it in large pieces. 

The siliceous variety of the ordinary brown sandstone may be seen 
in any house, where brownstone is used, in any large city. It has often 
been the case, that in examining houses where the decay has gone on to 
such a degree as to almost make it necessary to take the fronts down, 
certain of the stones that were composed either of pebbles, or of less 
coarse grains cemented by quartz, were still so sharp, having undergone 
no decomposition of any kind, that they could be put back into the 
building with perfect safety. This is true, not only of the facings of the 
building, but of the ornamental stonework around the porticoes of the 
houses. If the selection of only such stones as have a siliceous cement 
had been made from the quarry—as it undoubtedly was by the masons, 
who, at the time that the Cathedral of Rodez was built, were a religious 
order—we should probably have had little experience of the decay of 
sandstones. Those men selected from the quarries only such stone as 
they knew from tradition or their own experience was most suitable. It 
is a curious fact, and a standing monument to their knowledge, ability, 
skill and conscientious work, that many of the sandstone buildings 
which they constructed are still standing without serious decay, while 
those built later on, are either on the way to ruin, or are already in ruins. 

All of the different kinds of weathering on the brown sandstone can 
be distinctly seen in Trinity and St. Paul’s churchyards. There is 
every gradation of it, from the stone that shows no trace of ever having 
been cut, and is just crumbling to sand, to that which stands apparently 
as fresh as the day it was put up. That careful selection of the stone is 
all that is required to have it last, is shown by the fact that there is in 
Trinity churchyard a brownstone headstone, cut on both sides, bearing 
the date of 1681, which still shows the marks of the tools used to cut it. 
It is but little pitted, though it shows the marks of age. Others in St. 
Paul’s churchyard just as sharp, and showing also the tool marks, bear 
the dates of 1760, 1766, 1770, 1781, 1787, 1793, while some of 1733,* and 

* In St Michael’s churchyard. Charleston, S. C., there is a cypress-wood head-board, 
erected to the memory of Mary Anne Luten, who died in the year 1740, which is still in good 
preservation, and likely to last many years. 



34 


a little later, seem to have just started to crack, and many of later date 
have crumbled to sand. All of these stones stand vertical. That to the 
Rev. S. Johnston, President of Kings, now Columbia College, erected 
in 1758 on the north side of the tower, was so defaced that it was recut 
in 1883. The one to Mr. Faneuil, which lies flat on the north side of 
the church, which was erected in 1719, has the inscription still sharp. 
Most of those put up within this century have the dates barely visible. 
One sharp stone, erected in Trinity churchyard on the south side of the 
church, on Broadway, bears the date of 1746, and beside it is one of 
limestone, bearing the date of 1793, the inscription on which will last 
but a few years longer. Comparatively few of the stones erected after 
the commencement of this century are well preserved. 

The causes of the decay in all of these stones are the solution of the 
binding material, and consequent deterioration of the strength of the 
stone; or, where the stone is weak in some parts and strong in others, 
as in the case of many of the ferruginous sandstones, the part of the stone 
unable to bear the pressure yields, throwing the weight upon the rest of 
the stone, which having too great a pressure upon it also yields, and the 
stone fails entirely. Where the binding material is lime, the cause of the 
disintegration is the action upon it of the minute quantity of carbonic 
acid, and sulphurous and other gases, dissolved from the air in the rain 
water, which readily attack the surface of the stone. If the mouldings 
are improperly cut, so that the water remains on them for any length 
of time, or passes through them, it may be that the surfaces of the under 
sides of the projecting pieces, such as balconies or projecting sills, 
which are improperly drained, and places where either the snow and 
ice of winter or the water of ordinary showers may rest, so as to become 
either absorbed in the stone or to pass directly through it, carrying with 
them some of the binding material in solution, which eventually be¬ 
comes entirely dissolved, will become either partially or wholly disin¬ 
tegrated. Such is the case with the very thin mouldings which form 
the tops of the windows of the highly-decorated houses built at the 
time this sandstone was first introduced. 

The effect of the small amounts of city gases absorbed by the water 
in dissolving out the binding material of the stone, can be most dis¬ 
tinctly seen wherever there has been a leak which has caused a perma¬ 
nent drip over the surface of the stone. Here not only the change of 
color, but the pitting from the solution can be most distinctly seen. It 


36 


is as true in nature on a large scale as in the laboratory, that the lime 
and magnesia is less soluble in concentrated solutions than in diluted 
ones. The amount of carbonic acid in the water and the air at any time 
is extremely small, but the accumulation of very minute quantities, 
acting over a large area for a considerable length of time, has, in many 
instances, been sufficient to dissolve ont the whole of the binding ma¬ 
terial, leaving little else than a skeleton of sand. Where there are two 
binding materials, one of which is lime, the solution of the latter leaves 
the stone porous. When oxide of iron is the binding material, this is 
easily dissolved out, though not so easily as the carbonate of lime, unless 
the iron itself has come from the decomposition of pyrites, when it con¬ 
tains sulphur, in which case its oxidation would cause the solution of 
some of the binding material, which would also be acted on by the 
organic and mineral acids contained in the city gases. The decomposi¬ 
tion of some of the sandstones of the coal formation in Kentucky is 
both interesting and peculiar. These rocks are of variable texture, 
being in places coarse and forming conglomerates, while in others they 
resemble a very uniformly fine sandstone. When this fine-grained rock 
is first exposed, it is quite hard. It stands up against the weather on all 
sides except the north. On this side it disintegrates and falls to sand, 
in irregular caves, often 10 or 15 feet high, forming by its disintegration 
the only pine lands of the district. This cave forming takes place hor¬ 
izontally and vertically at the same time, leaving a thin hanging front, 
which to all appearances from the outside is solid rock. After some 
time, holes of irregular size form in this front, which eventually breaks 
down, as does also the overhanging rock, when it can no longer bear 
the weight above it. In this case the disintegration is caused by weak¬ 
ness from the loose consolidation of the grains of sand. 

The decomposition of the sandstones shows itself in several ways. The 
first and the most general one is by flaking, where large pieces, apparently 
parallel to the stratification chip off, to the depth sometimes of a quarter of 
an inch. When these are examined carefully, they do not always show any 
trace of following the strata of the stone. This flaking does not take place 
until the most of the binding material has gone, or it has been entirely 
dissolved out. Whether this material is of an organic or mineral nature, 
the depth of the decomposition will determine the point at which it 
flakes. The least blow, or a hard winter, or the infiltration of water 
which freezes, or the accidental arrangement of a series of mica plates. 


36 


will determine the point as well as the moment at which it will fall. 
Sometimes, when this cause exists only to a small extent, or when there 
is a cavity in the stone, it will begin to bulge away from the sides, 
and may even crack out by the infiltration of moisture, when that mois¬ 
ture is caught in the interior and freezes. This frequently takes place over 
roughly finished surfaces, as the projecting fronts over door-ways, which, 
when the phenomenon commences, sound hollow when struck, and are 
then entirely at the mercy of the frost, which will eventually break 
them to pieces. I have frequently seen slabs on the upper parts of 
stoops, five feet square, which were detached from the underlying 
stone in this way, in the center of a larger slab, which remained 
in this condition for several years, showing no trace of decompo¬ 
sition on the surface, and which were finally detached from it either 
by an accidental blow or by the heaving of the frost. When such slabs 
are once broken or cracked, the pieces rapidly flake off. It is asserted 
by some stonemasons that flaking may be produced by improperly dress¬ 
ing the stone; that where the surface is dressed by the blows of heavy ham¬ 
mers, particularly if a large number of blows are made in regular time by 
a uniform movement either of machine or hand, the surface of the stone 
becomes so fatigued that there is a tendency toward the separation of the 
grains, and that a stone so dressed will eventually laminate. Whether 
this be true or not, it would only account for a part of the difficulty. 
Although this method of dressing has been very generally abandoned 
for the planing and polishing machines, there is no diminution of the 
lamination of these stones. 

I have tried filling the cavities made by fracture out of a laminated 
surface in such stones, with stone putty or cement, but never have been 
able to keep them from breaking off on horizontal surfaces. I have 
sometimes been able to hold them in place for a few years with shellac on 
a vertical surface. That the decomposition has gone beyond the pieces 
flaked off may be seen by examining the surface of the stone from which 
the flaking has taken place with a glass, even when the fresh fracture 
has been exposed for but for a short time only. On looking carefully at 
it, it will be found that the surface is pitted. This pitting on the outside 
surface is generally taken for hammer marks, but on examining it care¬ 
fully it will be seen that the stone has become open and that it has 
changed color. This change of color is owing to the deposit of a certain 
amount of soot at the bottom of each one of these holes, so that the 


37 


change is an apparent and not a real one. This pitting not unfrequently 
extends through the flake to the surface of the stone below. If exam¬ 
ined carefully at this stage, it will be observed that the pitting is owing 
to the fact that the binding material has been dissolved out to a consid¬ 
erable extent. When such stones are examined with the mircroscope it 
will generally be found that the pitting, which is all that is at first 
visible, has been accompanied by a honeycombing of the stone, which 
appears as if riddled with minute holes when the pitting has gone deep 
enough, and that the binding material, of whatever nature it might have 
been, has been either partly or wholly removed. This is especially true 
when it is carbonate of lime, or when there has been any cause of weak¬ 
ness, such as the presence of minerals which have a tendency to assume 
specific directions. Any cause which will produce an unequal expansion 
such as the freezing of the water infiltrated, or the exposure of the sur¬ 
face to a very hot sun, will have a tendency to cause the stone to break off 
in thin lamellae, or to flake. Pitting as it usually occurs in the brown- 
stone of the city, cannot generally be seen by the eye, and can only be 
observed by a careful examination with a glass. Occasionally a thin 
flake of the decomposed stone is sufficiently honeycombed to allow the 
light to pass through the fine pin holes. Sometimes, however, as in 
the stone of Castle William, on Governor’s Island, it is quite deep, in 
some cases in rounded, but oftener in elongated holes, which have 
penetrated the stone a quarter of an inch or more. When the pittings 
are so large as this the stone does not flake, but falls into sand. 

Sometimes the stones do not flake, but simply crumble over the surface 
and at the sharp edges. This occurs where either the angles have been 
too sharp for the stone, or where the stone has not been properly under¬ 
cut, so that the water passing through dissolves out the binding material 
and Reaves nothing to hold it together. This kind of decomposition 
takes place in the quarry as well as in the structure, and may have taken 
place to such an extent that the stone is worthless to a very considerable 
depth. I have known of some cases when it was necessary to strip the 
sandstone of a quarry for 20 feet before arriving at any sound stone. 
It is said that in opening new sandstone quarries the rock for 2 to 3 
feet can be removed with a pick, and that it is generally necessary 
to reject the stone for several feet. It is, therefore, not safe to 
use a stone from a new quarry without first making a complete 
microscopic examination of it, which should be supplemented by a 


3S 


partially chemical one, to ascertain first whether the stone is sound; 
secondly, whether it contains any material which will be likely to be 
either decomposed or be absorbed by the action of the elements. With 
the sandstones that are very porous and have lain in the quarry bed, the 
mere freezing of the water absorbed will be sufficient to flake off part 
of the surface; or, when that does not take place, to crumble the stone 
considerably. 

It was formerly the practice to require by contract that the stone 
should be exposed to the air for a certain time after being taken 
from the quarry to season; that is, have the quarry water dried out 
of it, and to ascertain whether there was any commencement of de¬ 
composition or disintegration in any part of the stone. This was 
the practice of most of the ancient architects. Sir Christopher Wren 
required that the stone for St. Paul’s Cathedral, in London, should 
season for two years after it was taken from the quarry, before it was put 
into the building. It is well known that stones with the quarry water 
in them are much softer than after exposure. This is particularly true 
of the soft limestones of Paris, which can be easily cut with chisel and 
saw ’and planed with a plane while the quarry water is in them, but 
which afterwards become quite hard, so as to very considerably in¬ 
crease the expense of dressing them. It has been frequently remarked 
that when stones with the quarry water in them are exposed to the tem¬ 
perature at which the water freezes, they chip much more readily than 
when, after it has once been dried out, they are exposed to moisture. 
The explanation which has been given for this is that the quarry 
water contains salts in solution, which, on the evaporation of the water, 
bind the stone together, and thus make it stronger. Whether this is so 
or not, it was well known to the ancients, and it has been shown by mod¬ 
ern experience that a tempered stone lasts longer than a fresh one, and 
the fact merits the attention of builders. It often happens that decom¬ 
position may have gone on in certain localities to the depth of three to 
four inches, without its being perceptible on the surface. Where iron 
pyrites, in any shape in which it can decompose, is present, this will act 
upon the various kinds of bindiDg material, and cause the stone to disin¬ 
tegrate. 

The causes of decay then are: first, improper selection of the stones in 
the quarry; almost every sandstone quarry contains poor stone, as well as 
that which will last for a great length of time, and this can always be dis- 


39 


tinguished because it is highly siliceous, and contains no traces of mica, 
and but little material soluble in acids. The presence of mica is essen¬ 
tially bad because it shows a probable tendency to lamination in the stone. 
Where the stone would otherwise be good, the improper cutting of mould¬ 
ings, and especially the want of undercutting of projecting mouldings, so 
that the water will be shed instead of running up over or down through, 
is the most fruitful cause of decomposition in the buildings of the present 
day. Where the edges are very sharp, there will always be a tendency, by 
capillary attraction, for the water to run up until it meets some point so 

ascending that it cannot follow up, and from here it will drip without 
\ 

passing through the stone. Where the surfaces are flat, crumbling 
or lamination will take place; where they are thin, so that the water 
will pass through them, the stones will crumble and flake; and 
where there are large surfaces immediately under projecting mould¬ 
ings, in such a condition that the water will flow up over the mould¬ 
ing, and then slowly down the face, lamination will take place. 
It is a remarkable fact, in looking over buildings that have undergone 
this decomposition, that the greater part of it will be found within a com¬ 
paratively short distance from the ground—generally within ten feet. 
From this point it gradually diminishes up to about 50 feet, where it ceases 
entirely, unless there is some action of sulphurous gases, or the acid or alka¬ 
line gases from the combustion of wood or coal, or from manufactories, 
coming in contact with the stone. In all the buildings that I have had 
occasion to examine, beyond one hundred feet there is not only no de¬ 
composition, but the stone is harder than it appears to have been origin¬ 
ally. The same stone which would last almost indefinitely when put 
into a building in the open country, with no other buildings near it, 
would be subject to decay in the city. The present mania for high 
buildings, if it had not been stopped by law, would have increased this 
decomposition, and would have greatly raised the maximum height to 
which it would reach. 

The stone of which Trinity Church is built is a red sandstone, which 
has always been supposed to be homogeneous, and, until the decay which 
has become manifest within a few years, has always been supposed to be 
one of the stones that would resist the action of the weather. The investi¬ 
gation which has been made,* has shown not only that there are different 

* The microscopic examination of the specimens, taken from Trinity Church, was made 
by Dr. A. A. Julien, of the School of Mihes. 




40 


kinds of stone used in the building, but that none of the varieties are, 
strictly speaking, homogeneous; for, instead of being composed entirely 
of quartz grains, united by a cement, they contain not less than twenty- 
eight different varieties of minerals, embracing twenty-five species, some 
of which are quite susceptible of decomposition, and many of which have 
begun to decay in places, and this process is still going on. The fol¬ 
lowing is the list of the minerals found in the sandstone of Trinity 
Church : 


Quartz, 

Biotite, 

Calcice, 

Amphibole, 

Hydro-biotite, 

Dolomite, 

Epidote, 

Sericite, 

Magnetite, 

Garnet, 

Margarodite, 

Hematite, 

Oligoclase, 

Fibrolite, 

Limonite, 

Orthoclase, 

Brown tourmaline, 

Gothite, 

Microcline, 

Green tourmaline, 

Pyrolusite, 

Kaolin, 

Indicolite, 

Rutile, 

Muscovite, 

Cyanite, 

Asphalt. 


Apatite. 



In addition to these, there is a considerable quantity of opacite, a 
name given by Vogelsang to the black opaque grains and scales which 
so frequently occur in rocks, and which cannot be identified with mag- 
nitite, menaccanite or any other mineral. What is more remarkable in 
this enumeration of species is the complete absence of pyrites. All of 
this stone contains a considerable quantity of material easily soluble in 
acids,* most of it effervescing, a certain portion of it going into solution 
with the greatest ease. If the stone had been properly selected at the 
time of the building of the church, or if the mouldings had been so con¬ 
structed as to shed water, I think there would have been but little de¬ 
composition at this time, but without some protection it must eventually 
have taken place. 

The depth and kind of decomposition differ in each variety of stone, 
but all have undergone more or less of it, and wherever any of the stone 
lies below a projecting piece, the decomposition has gone on to a very 
considerable extent. After selecting the pieces that were more or less 
affected, I succeeded in getting a piece of the stone which had been 
lying exposed to the weather until within four or five years, and had 
then been accidentally buried. I had it dug up and left exposed to the 


41 


air for some time, and then carried to the cellar of the church, where it 
was dried at a temperature of about 60 degrees for two or three months 
before the specimens for examination were taken from it. 

The decomposition shows itself in these stones in three different 
ways : first, by deep pitting and falling out of grains of the minerals 
which compose the 'stone; second, by lamination; and third, by crum¬ 
bling to powder. The way in which each one of these is effected depends 
upon the position of the stone. The pitting is quite irregular, and is 
due to the removal of the cementing material from the surface; and 
where this has been done by the action of the weather, there seems to 
have been a deposit of sooty material at the bottom of the depression 
which has slightly changed the color of the stone, so that the outside is 
of a different tint from the inside. The cause of this is not shown until 
the stone is examined with a microscope of high power. On treating 
such a stone as this with acid, generally no effervescence takes place, and 
very little of it is soluble. The same is true, but to a less extent, where 
the rock has flaked. The flaking does not appear to be owing to the 
fact that the stone has been used in a different position from that which 
it held in its quarry-bed, though in many places it has evidently not 
been placed in its natural position; nor to the fact that the stone is lam¬ 
inated, owing to the presence of large quantities of mica; but it is owing 
to the absorption of the cementing material by atmospheric influence. 
It is also to be observed that there are in many sandstones which are 
used for building in this city, organic materials which act as the cement, 
and are easily decomposed by exposure to the weather. Stone of this 
kind, except to a very limited extent, does not seem to have been used in 
the walls of Trinity Church, though it is extremely common in other 
parts of the city. It can only be kept from complete disintegration by 
filling the pores of the stone with some substance which will prevent 
further encroachments of the weather, which must be renewed from 
time to time as it is decomposed or dissolved out. By the original 
agreement, the stone of which the church was built was to be taken 
from a single quarry, which had been determined by observation to give 
a strong stone,'but this agreement, like that made for the stone of the 
House of Commons, in London, was not adhered to. The different 
varieties of stone used in the building are: 

First. —Fine-grained, lamellar reddish-gray. Of this variety there 
were 15 specimens. 

k 


42 


Second. —Fine-grained reddish-brown. Of this variety there were 6 
specimens. 

Third. —Fine-grained light gray. Of this variety there were 6 
specimens. 

Fourth .—Coarse yellowish-gray. Of this variety there were 3 speci¬ 
mens. 

While the varieties examined were most of them weathered, specimens 
of these varieties are easily distinguishable in the unaltered stones of the 
building itself. The stone was, for the most part, of the same origin, having 
had in the quarry a calcareous cement. It does not appear that any large 
quantity of this stone would have endured exposure in the quarry or 
in the building for any great number of years without being affected. 
None of those which had a siliceous cement appear to have been decom¬ 
posed. There are two types of the first variety of stone, distinguished 
as A and B. Both of these were taken from the piece that had not 
been exposed in the building. The type A was coarsely laminated and 
contained the following minerals: Quartz in very irregular grains, most 
of them rounded, some of them quite spherical, filled with minute 
bubbles either in motion or quiet, varying in color from smoky, through 
white to red; orthoclase, plagioclase, probably oligoclase, and microcline 
distinctly cleavable. These were the prevalent minerals. Magnetite 
and menaccanite in grains of varying size are quite evenly distributed. 
Biotite, hydro-biotite, muscovite and margarodite are quite regularly 
scattered through the rock; epidote in small particles, rarely in prisms; 
green and blue tourmaline in occasional grains; iron ochre and garnet 
in very small grains, and sericite in minute bunches around and between 
the grains of quartz and feldspar, are also found. Of all the minerals 
the micas are those which have undergone the most decomposition, the 
hydrated varieties appearing everywhere, and these frequently having 
become opaque from still further decay. The rock gave a lively 
effervescence with citric acid, showing the presence of calcite; apatite 
was also present in small quantities. 

The composition was determined as 

Q^tz. 57 p ar t s . 

Feldspar.•. 31 

Fine-grained minerals. . . 12 “ 

100 





43 


The rock was slightly altered, as were also the minerals that compose 
it. To give a fair idea of the amount of decomposition, the grains of 
about the same size were counted and classified according as they were 
more or less affected. 

Clear. Slightly Cloudy. Very Cloudy 


or Opaque. 

Orthoclase. 6 22 16 

Oligoclase. 4 1 

Microcline. 1 

Hydro-biotite. 14 14 12 


It thus appears that the feldspars were the most decomposed. 

The second variety of the stone, B, which had not been put into 
the building, had lain on the ground in the churchyard exposed to the 
elements for about 30 years. One part had been acted on by the 
weather, and was of lighter color than the rest, owing to pitting. Here 
the dropping out of grains of quartz and feldspar is distinctly visible, 
but all parts of the stone effervesced with citric acid. The specimen 
contained about the same quantity of quartz and feldspar as No. 1. 
Hydro-biotite, sericite, margarodite and amphibole are in elongated and 
crushed fragments; kyanite, blue tourmaline, hematite, magnetite, 
limonite and calcite were also found. 

The way the alteration of the minerals which compose the rock had 
taken place, is given below: 

Clear. Cloudy. Very Cloudy. 


Orthoclase. 9 22 4 

Oligoclase. 11 2 

Microcline. 1 1 

Hydro-biotite. 3 1 2 


The weathered part of this variety is deeply pitted, but there does 
not appear to be any appreciable widening of the distances between the 
grains, but the rock is honeycombed by the deeper of the pits going 
through the flaking; the calcite was entirely gone, no effervescence 
taking place in citric acid. The minerals themselves do not seem to 
have undergone any decomposition since they became incorporated 
in the stone. 

Of the second variety there were two unaltered specimens, both of 
which were taken from the inside of the tower high up in the steeple. 
It contains much more ochre than the other varieties. The minerals 












44 

composing it are quartz and feldspar as before, brown tourmaline, seri- 
cite, margarodite, hydro-biotite, garnet, epidote and magnetite. It does 
not effervesce. 

The proportion of the ingredients contained in the stone was: 

Quartz .. 

Feldspar 

Cement.. 

All the crusts are deeply pitted and honeycombed. 

The third variety is fine-grained and indistinctly lamellar. The 
fresh specimen was taken high up from the inside of the tower. It 
effervesces rapidly with citric acid. The most prominent mineral is 
quartz, which is quite clear; the orthoclase is generally cloudy. The 
accompanying minerals are amphibole, garnet, margarodite, hydro- 
biotite, brown tourmaline and sericite. There is but little plagioclase. 
The following proportions of these minerals were found in three speci¬ 
mens: 

1 . 2 . 3 . 


Quartz. . 53 61 51 

Orthoclase. 22 17 39.5 

Hydro-biotite. 2 0.5 

Cement. 25 20 8.5 


100 100 , 99.5 

The feldspar is whitish, the cloudiness being caused by a great 
abundance of microlites generally arranged in parallel planes about 
normal to the direction of easiest cleavage of the feldspar. Between 
these cleavages there is a slight transparent film of sericite which po¬ 
larizes feebly. This mineral is probably the result of the commence¬ 
ment of the alteration of the feldspar, but in some of the grains the 
decomposition has not progressed further, while in others its progress 
is shown by the innumerable very small microlites which are quite 
colorless, but give a milky appearance to the grains where they occur. 
They are probably both sericite and epidote. The hydro-biotite is 
transparent green in the center, and opaque brown on the edges, show¬ 
ing a commencement of decomposition. The other minerals, margaro¬ 
dite, sericite, epidote, tourmaline, iron ochre, do not show anything 
peculiar. Where the specimen is not decomposed it is close-grained 
without interstices, when it is weathered it is honeycombed. 


41 

35 

24 









45 


The fourth variety was coarse-grained yellowish. It is quite com¬ 
pact, so that little pitting is found on it. It contains no carbonate of 
lime. The quartz is in closely-packed grains, generally clear, but con¬ 
taining some inclusions. The grains are angular. The orthoclase is 
also angular, some of the grains being cloudy. The plagioclase is also 
angular, and polarizes the light strongly. The biotite and hydro-biotite 
are in yellowish-green scales. The other minerals are margarodite, 
epidote, amphibole, sericite, magnetite and iron ochre. No pitting 
occurs, except in the specimens in which there are red flakes, and here 
it is quite deep. The specimens where this occurs were taken from the 
outside of the church near the ground, where they were exposed not 
only to the action of the weather, but to that of the dampness coming 
from the ground as well. The composition of these specimens is given 


below : 

1. 2. 3. Average. 

Quartz. 40 70 57 56 

Orthoclase. 53 18 31 34 

Cement. 7 12 12 10 


With regard to all of the stones, it may be said that the disintegra¬ 
tion is not caused by either the looseness of aggregation of the par¬ 
ticles, nor by the structure of the stone. In all the specimens there 
was shown great compactness and no cavities, except occasionally when 
grains had fallen out owing to the absorption of the binding material. 
It was not influenced by structure, for while there is an abundance of 
mica, the splitting is as much across the mica plane as in it. If any¬ 
thing, the mica, though acted on, seems to have played, to some extent, 
the part of a binding material. 

The disintegration is not owing to the decomposition of the feldspar, 
most of which belongs to the variety orthoclase, which decomposes with 
comparative ease. It is to be noticed that the average amount of ortho¬ 
clase in the specimens examined was between 29 and 30 per cent. Most 
of these grains were more or less decomposed, but there was no evidence 
to show that the decomposition has progressed to any extent since the 
consolidation of the sand into stone. On the contrary, it seems to 
have been completely arrested. This is shown by the fact, that in the 
most highly-weathered specimens, the proportion of milky grains of 
feldspar showing inclusions and the commencement of decomposition 
is no greater, and the decomposition no further advanced than in the 





fresh ones. The stone seems to have been formed out of the destruc¬ 
tion of granitic rocks, which had commenced to decompose during the 
trias previous to their destruction, and to have been consolidated under 
immense pressure, as is shown by the crumpled and irregularly broken 
and curved mica plates, and the compact nature and close union of the 
grains. 

It appears, however, that some decomposition has actually taken 
place in the minerals of the stone since its consolidation into rock form. 
This is shown in the condition of the iron micas, both the biotite and 
hydro-biotite, which contains from 5 to 25 per cent, of ferrous oxide, 
from 1 to 4 per cent of water, which readily decompose. That some de¬ 
composition has taken place in it is shown by the varying color of the 
mineral, individual pieces being mottled brown and green, and also by 
the action of polarized light upon them, as well as by the fact that their 
passage into chlorite and iron ochre can be distinctly seen. Part of this 
decomposition is seen in the unaltered stones, but it is much more 
prominent in the weathered ones, and while of itself it would have had 
but little influence in weakening the stone, it undoubtedly put the con¬ 
stituents into a form in which they would be more easily attacked. A 
very slight alteration in the margarodite, amphibole and garnet is seen. 
While this decomposition is not sufficient to account for the entire 
weathering of the stone, it undoubtedly contributed, although slightly, 
to it, as at the most it could weaken the stone but little. It is, how¬ 
ever, to be observed that the mineral calcite is present in every one of 
the unaltered specimens except the last variety, which consisted of three 
specimens out of the thirty examined, and that in all the weathered 
varieties this was either wanting entirely or present in very small quan¬ 
tities, and that wherever it was dissolved out to a small extent, there the 
stone was pitted, and when entirely gone honeycombed, rendering that 
part of the stone porous and open on the outside. How far the iron 
has been removed at the same time as the lime, cannot be stated. 
There has been no very great change in the color of the stone—cer¬ 
tainly not sufficient for us to suppose the removal of any large quantity 
of iron oxide, and but little that may not be accounted for by the pres¬ 
ence of small quantities of carbon deposited at the bottom of the pit- 
tings in the stone. The decay is deep-seated, penetrating, at times, 
several inches into the stone, which still looks sound on the out¬ 
side, not showing the least trace of flaking; but in no case is the lime 


47 


entirely wanting from tlie interior of the stone, even when it has been 
entirely dissolved from the surface. The removal of even a very small 
quantity of lime would render the ochreous material which is so abund¬ 
ant much less stable, and far more likely to be acted on. It is not very 
homogeneous in any part of the stone, but is much less so where the 
calcite is gone. This action on the iron, while it could not go on at any 
great depth from the surface, and so could not weaken the stone much 
in the interior, was at its maximum on the outside of the stone; and, as 
it was already porous from the solution of part of its binding material, 
this rendered it likely to flake at short distances from the surface, the 
stone falling off in layers along the lines of least resistance, correspond¬ 
ing to the plane which included the greatest number of pit bottoms, 
and consequently give an appearance of lamination in the stone which 
did not exist in the stone itself. Frost would separate such stone into 
lamellae very rapidly. 

Independent of all these considerations, is the fact that there is in all 
these stones a certain amount of organic matter, which can easily be 
detected on heating the stone, by the empyreumatic odor which it gives 
out. The microscope does not show any vegetable matter, nor, in most 
of the stone, any traces of carbon which is not in a state of combination. 
It is well known that organic acids form combinations with bases, which 
combinations are decomposed when submitted to the continued action 
of moisture, and will be all the more rapid as this moisture is slightly 
acid, as it is in all cities. It is most common to find, either in or in the 
vicinity of these rocks, large quantities of organic materials, either fos¬ 
silized, such as plants, lignite, coal, asphalt, or combined with bases. 
Asphalt was actually found, in very small quantities, in two specimens 
of undecomposed stone. In other sandstones organic acids have been 
found. It is more than probable that they would be found much oftener 
if looked for. The expense of such investigations, however, generally 
prevents their being made. Dr. Julian has called attention to the fact 
that the ochreous condition of sandstones of different geological ages is 
not the same. In those of the paleozoic periods it is hematite, which 
frequently produces such a permanent cement as to make a durable 
building-stone. In the tertiary it is turgite or limonite, and in so small 
quantity as to jjroduce in many cases only sand rocks. I once had occa¬ 
sion in the Far West, to examine the abutments of a railroad bridge 
built of stone excavated in the winter from such a quarry, and 


found that the rock, though apparently strong, could be rubbed into 
sand with the fingers. In the sandstones of intermediate geological 
ages, gothite predominates, and is in variable quantities in different 
layers of the same bed, making the rock correspondingly strong or 
weak. The range of the variation is so great as to imply the possible 
association of organic acids with the iron oxide. Whatever may be the 
cause of the decay, it is not loose consolidation of the rock. It is not 
wholly explained by the solution of the calcareous matter, but is 
undoubtedly hastened by it. It had progressed somewhat in the quarry 
before the stone was put into the church, and is still going on. The 
flaking of the stone in thin parallel layers is perfectly explained by the 
solution of the lime, but the deep-seated decay needs further study, as 
the action of the acid, waters and gases does not fully explain it. The 
flaking is hastened by the heat of summer and the frost of winter, and 
is always most prominent where there is any drip of water. 

It is certain that the decay can be retarded, even in the buildings 
which were constructed twenty or thirty years ago, so as to make them 
last for many years. It is not certain that it can be completely arrested. 
The decay in Trinity Church is one of the best examples that could 
have been chosen, for it applies to one of the most compact and well- 
selected building stones of the country, which was selected at the time 
on account of its supposed great durability. It is certain that from 
the same quarry, by a careful selection of stones with siliceous binding 
materials, and the rejection of all others, material might have been 
selected that would have lasted indefinitely. 

It has been usual to consider all sandstones as good for building 
purposes, or not, according as they were compact or more or less 
porous, and this rule has undoubtedly influenced the choice of the stone 
which has been used, which is in this respect an extremely good one. 
The theory generally accepted is that the porous stones absorb moisture, 
which in the cold of winter freezes and, expanding, disintegrates the 
stone; and that the heat of the sun also contributes its share to the ex¬ 
pansion, which causes the flaking. This cannot be said to be true to any 
great extent of the stone of Trinity Church, except where the moisture, 
from the defective cutting of the mouldings, has not been shed from the 
surface, and has been constantly dripping on the stones below. It is 
specially observable that on all the faces of the building underneath pro¬ 
jecting stone, whether it has had mouldings cut on it or not, the stone 


49 


has decomposed more or less. It is a remarkable fact that in all of the 
specimens taken from all parts of the chnrch, both inside and out, and 
representing so many varieties of stone, no iron pyrites has been found, 
and consequently no such cause of disintegration as often produces 
decay in other building matei’ials can be ascribed to it. 

The microscopic examination revealed the fact that a considerable 
quantity of carbonate of lime was contained in most of the specimens, 
the fresh stone containing the most; and that in all the decayed crusts 
the lime was present in very small quantities only, or not at all; and in 
many cases this decomposition has gone on to some extent, even to the 
depth of eight or ten inches in the interior of the stone, without show¬ 
ing any trace of decomposition on the outside. The stone was thus 
rendered extremely porous by this solution, even where the weathered 
exterior retained the smooth surface of the original dressing. It there¬ 
fore was porous in the interior as well on the surface, and was subject to 
decomposition in all its parts. This is shown to take place far into the 
interior, even when it appeared not to be affected on the outside at all. 
I was not able to find a single specimen of the stone in any part 
of the building that had not apparently undergone this action to a cer¬ 
tain extent. In some few cases the pitting is owing to the dissolving 
out of a small amount of the oxide of iron, which serves the purpose of 
an additional cement; but it is generally the carbonate of lime which 
has been removed. 

In every case—and but few such were noticed—where the stone has 
been set upon its edge, the lamination has gone on to a considerable ex¬ 
tent; but what is more remarkable than this, is the fact that all of this 
stone in every part has undergone more or less of this interior decay, 
which seems to extend through the whole stone, and in some places 
where it was particularly weak has caused the bulging of the face in thin 
lamime, even where the stone has been laid in its quarry-bed. In a very 
few instances it seems that this bulging may have been owing to a de¬ 
composition of some of the minerals contained in the stone. 

None of the causes which are usually alleged to account for decay 
seem to be prevalent here. These are usually a loose consolidation of 
the sand of the stone, owing to the fact that the cementing material is not 
quartz, as it is in the Potsdam sandstone, used in the new buildings of 
Columbia College, but is some material subject to decomposition. The 
true cause of the decay seems to be the fact that, owing to the defect 


50 


of the undercutting of the mouldings where there is a drip, the water 
remains in and is absorbed by the stone, and in some cases passes 
through it. Every shower of rain, particularly in cities, removes a very 
minute amount of binding material from the outside. When the stone 
is soft, owing to the small quantity of binding material contained in it, 
the action is comparatively rapid; and if to natural causes are added the 
corrosive gases of the city, decay will be very rapid. This decomposition 
goes on most rapidly under projecting horizontal pieces. The flow of 
rain water over a vertical surface of stone injures it but slightly, only 
that which is absorbed affecting any damage. The rest is so rapidly 
shed that it has no time to dissolve out anything. When the water is 
rapidly shed, the solution of the binding material is uniformly slight, 
as the evaporation of the water leaves all the solid parts which it may 
have dissolved behind. But when the water passes through the stone, 
or the quantity dripping on it is such that the stone cannot absorb it, 
so that it either passes directly through or runs off from the surface in 
more or a less of a stream, then the quantity actually dissolved out will 
always be a maximum. It is to be remarked that the places where decay 
exists are precisely in these conditions. In the country this would 
probably have but little effect, and the building would last a long 
time; but in the city air, which is charged with a larger quantity of 
carbonic acid than is usual in country air, and also with small quantities 
of sulphurous and sulphuric acids, and with all the acid gases that 
result from the decomposition of city refuse, which are absorbed by 
the water, it has the power of dissolving the cement of the stone even 
though in minute quantities, and, after a considerable period renders 
the stone so loose that after a time it will flake and fall to powder, or 
become disfigured. As soon as this state of things was suspected, a 
careful chemical examination was made of five of the stones of the 
church taken at random, selecting, however, one No. 1, which was 
fresh stone. 

The analyses of these stones is given below. They show that in the 
weathered specimen a large quantity of the binding material has been 
dissolved out, leaving the stone in a condition to be acted upon by all 
the decomposing agencies of the weather, as frost, expansion from heat, 
expansion from cold, and the tendency which decay always has to spread 
when it has once begun. 


51 



No. 1.* 

No. 2. 

No. 7. 

% 

c 

H 

C 

• 

No. 21 


Fresh unex¬ 
posed stone. 

Weathered. 
Outside of No. 1. 

North side tower 
base and but¬ 
tresses. 

South side be¬ 
tween 6th and 
7th buttresses. 

South side base 
between 4th and 
5th buttresses 


Interior. 


Scales. 

Scales. 

Scales. 

Insoluble. 

90.30 

91.64 

96.09 

95.82 

94.27 



Soluble. 




Iron oxide. 

2.25 

2.66 

2.32 

2.39 

1.97 

Lime. 

3.61 

2.45 

0.26 

0.14 

1.51 

Magnesia. 

Carbonic acid, 

0.30 

0.36 

0.28 

0.27 

0.20 

organic matter, 

etc. 

3.54 

2.89 

1.05 

1.38 

2.05 


In all of these stones there is a small amount of organic matter which 
gives a peculiar, smell when burned. In many of them no trace of 
effervescence with acetic acid could be seen under the microscope, and 
after careful drying in an air bath the total amount of soluble material 
did not exceed three per cent. 

The examination shows that there is no cause of decomposition in the 
stone itself; that the decomposition has gone on in all parts where the 
stone-has been exposed to the air, whether inside or outside, with regu- 
lariky^fhat most of the stone in the interior or at a certain distance removed 
from the surface is in about the same condition in which it was in the 
quarry; that the mere action of moisture or of the air affects the stone 
very slightly, when there is only a vertical wall to be acted on. The de¬ 
cay of the stone is, therefore, ow ing to causes which act from the outside 
entirely, and it remains now to consider what will be the best method of 
adding something from the outside of the church which "will prevent 
further attack upon and consequent decomposition of the stone. This is 
a matter of great importance, for, until the flaking of the stone, it was 
always supposed that the stone of this church would not decompose. 
The greatest care was thought to have been taken in its selection, but 
the investigation shows that the solution of the ingredients of the stone 
of one of the best buildings of the country, from the outside, has gone 
on to such an extent as to cause the stone to decay very perceptibly in 

* These analyses were made by J. B. Mackintosh! E. M» ,of the School of Mines. 









52 


less than forty years.* I am glad to be able to call attention to the prob¬ 
able solution of the problem, which, if not soon solved, will make most 
of the handsome edifices of New York ruins in the course of a few' 
years. 

The methods for the preservation of stone are, first, properly in¬ 
clined surfaces that will shed the water so that it cannot stand on the 
surface; properly undercut mouldings, so that the water cannot creep up 
under and remain so long as to dissolve out the binding material; keep¬ 
ing out the water as far as practicable. This must be done by water¬ 
proofing, so to speak, the foundations: placing a thin layer of asphalt 
between the foundation stones and the vertical walls of the building 
proper, 2 or 3 feet above the ground. This would prevent the slightly 
acid waters from the earth rising and decomposing the lower tiers of 
stones. Asphalting the fronts of the foundation walls themselves, 
previous to throwing the earth back against them, would prevent the 
entrance of a considerable amount of water, so that if the face of the 
foundation next the soil and its top were asphalted, the water would not 
enter here. 

If the stone has been already acted on, and is to be preserved, some¬ 
thing must be added to the outside to fill up the pores formed by the 
solution of the binding material, and prevent further encroachment of 
the water containing the acid gases in solution. To heat the surface of 
the stone, already weakened by decay, is only to make the decomposed 
surface more liable to flake. It must, therefore, be applied to the cold 
stone. Anything that forms a gum that is impervious to water, and can 
be made sufficiently liquid to penetrate the pores and pittings of the 
stone, will answer the purpose. Bees-wax or rosin dissolved in any of 
their solvents, with or without oil have been used. Boiled linseed oil, 
which forms a gum, does very well, and only slightly discolors the stone 
for a time. But all of these substances decompose after a few years, 
and must be renewed. Paraffin dissolved in boiled oil, and put on hot, 
answers better, as it is more effectual, and after one or two applications 
will fill the pores completely. But no substance applied after decay has 
begun is equal to a preventive and no preventive can preserve flat 
surfaces and projecting mouldings, which are not undercut, from decay. 
Water glass associated with a bituminous substance, a preparation 
known as Szerelmy’s compound, w r as used on some of the interior 


* All the decayed surfaces of the stone of Trinity Church were recut in the summer of 1884. 



53 


courts of the Houses of Parliament some years ago. I inspected all of 
these courts in the summer of 1884. The dilapidated condition not 
only of the finials, but even of the flat surfaces of the stone, showed 
that it had not even been a palliative. 

About the year 1868, the Vielle Montagne Co., of Liege, introduced 
the process of painting with water glass and oxide of zinc. This was 
applied to the railroad station at Liege, and to parts of the Houses of 
Parliament. A silicate of zinc and lime is thus formed on the outside 
of the stone, which indurates it superficially. The want of penetration, 
or careless application of the material, makes the surface flake, and it, 
like all other paints, requires frequent repetition. 

Ransome’s process, which consists of using water glass, with a sub¬ 
sequent application of some chloride, answers very well, but is a very 
expensive process, as it requires that the stone should be entirely re¬ 
faced, in order to clean it. This and the number of applications of the 
silicate, and subsequent application of the chloride, consumes so much 
time, and is so expensive, that it cannot come into general use. It has been 
applied satisfactorily to some small parts of the Houses of Parliament, 
but has been discontinued on account of its very great cost. The same is 
true of a number of other processes, which require first, the cleaning of 
the stone, and the subsequent application of several chemical substances, 
such, for instance, as the process of the Silicate Paint Co. They are 
excellent in themselves; they can be put on with certainty, when used 
with great care, and over small surfaces, but when they are to be ap¬ 
plied to buildings already constructed, they either fail from the impos¬ 
sibility of applying them evenly on large surfaces, or become entirely 
impracticable on account of the expense. The principle of most of these 
processes is correct, and it is to be hoped that some corporation will 
find it to their advantage to have investigations made that will lead to 
the discovery of some substance which is both cheap and of easy ap¬ 
plication. Such substances undoubtedly exist, they require to be 
sought for, but any process which requires the formation of a chemical 
union between the stone, and the substance applied must be put on in such 
a way as both to be certain of its penetration, so that the action will not 
be superficial, and to insure that the compound formed will not only 
become sufficiently indurated to withstand the weather, but sufficiently 
compact not only to resist the penetration of gas, but also to prevent 
the absorption o^ moisture, and to shed it at once from its surface. 


54 


Very few natural stones even do this, and they are the very compact 
siliceous stones, or dense carbonates, which are very homogeneous. 

When oils, with or without paraffine and sulphur, are used for water¬ 
proofing stone, they should always be applied while hot, as they are 
then much more liquid, and, consequently, more effectual. It is better 
also that the application should be made when the stone is also 
warmed by the heat of the sun—that is, either in the spring, 
summer or fall, rather than in the winter. This application of oil, 
however is not permanent in its effects. The oil forms a gum in 
the pores of the stone, filling them up temporarily, so that the water 
does not enter, but after a time this decomposes, and must be replaced. 
It is very easy to distinguish the surfaces of stones that have been 
treated with oil, by observing them immediately after a shower, when 
it will be seen that the stone becomes dry much more rapidly than that 
which has had no such application. The necessity of applying the oil 
once in every two or three years is one reason why it is so little used. 
When paint is used, the solid material with the oil is of no account 
whatever. It remains on the outside, and does not enter the pores of 
the stone, and when the gum is decomposed it is washed off the surface. 
Another objection to painting the stone, is that while adding considerably 
to the expense of the process for protecting the stone, it adds nothing 
whatever to the efficiency of the oil, and it is very apt to fill up the fine 
tracery that may be upon the surface of the stone. It is, therefore, entirely 
unnecessary, and in some cases may be injurious, as preventing the pene¬ 
tration of the material which is designed to water-proof the stone. When 
oil is heated with an excess of sulphur it dissolves about 13 per cent, of it. 
More of it will dissolve in hot oil, but the sulphur crystallizes out upon 
cooling, and when there is a large excess of it, a partial decomposition 
takes place, and sulphureted hydrogen is formed. The oil, with the 
proper amount of sulphur, becomes thick and dark like molasses, but 
when heated is quite thin. When the stone is coated with this material 
it penetrates below the surface, and as the sulphur is simply in solution, 
when the gum of the oil decomposes it leaves the sulphur still in the 
pores. Two or three applications of such a material as this would pre¬ 
vent any further decomposition. It has been found by experiments 
made on the Houses of Parliament that sulphur applied in some such 
way has been the only thing that has arrested decomposition in that soft, 
porous stone; but even this preparation has not prevented the flaking of 


55 


the surface of the stone after an exposure of about twenty years, prob- 
ably because it was not applied hot, and was put on when the stone was 
moist, or when the weather was cold. 

If 20 per cent, of paraffine be added to oil containing the sulphur 
in solution, it thickens when cold into a semi-solid buttery mass, but is 
fluid when hot. Such a preparation as this, applied hot to the stone, 
gives another element with the sulphur, which does not decompose, to 
fill the pores of the stone. There may be cases in which it is undesira¬ 
ble and inconvenient to use sulphur, and in these cases 20 per cent, of 
paraffine mixed with hot oil may be used. The same is true of this 
as of the other preparation mentioned. The paraffine, which is, like the 
sulphur, practically indestructible, remains in the pores of the stone 
after the decomposition of the organic matter of the oil has taken place, 
and one or two coatings of such material as this will waterproof the stone 
entirely and prevent further disintegration. Other preparations of paraf¬ 
fine may be used to advantage, but those which involve heating the surface 
of the stone should be avoided. It has been suggested that these prepara¬ 
tions discolor the stone. It is true that the color of the stone is darken¬ 
ed by them, but the stone is not disfigured, as the color given is very 
nearly uniform over the whole surface. In some cases the stones, after 
the application of the oil, have been washed with ammonia salts to take 
off the excess of the material from the surface. It is generally, how¬ 
ever, undesirable to do anything to the stone after the liquid has been 
applied, as the surface will very soon become bleached by the action of 
the weather. 

It has been objected to the use of oils that they would interfere with 
the adhesion of the mortar in the stone, if the stone were treated before 
it was put into the building. In actual practice, however, this does not 
seem to be the case. The stone is not only more thoroughly protected 
from moisture upon all sides, for in stone buildings in damp countries 
the moisture is as much to be feared on the inside as on the outside, but it 
does not seem in any way to deleteriously affect the binding power of the 
mortar. It acts advantageously both on the stone and the mortar, and does 
not influence any of the chemical changes that take place, either by ac¬ 
celerating, retarding or preventing them. This fact was well known to the 
ancients, for they used oil in mortars and cements to a considerable ex¬ 
tent. When it is desirable to use soft and very porous stones for build¬ 
ing purposes, the stone should be dipped either into boiling oil or some 


56 


such preparation as this, before being put into the building, so that its 
entire surface may be coated. The experience of the last thirty years 
shows that the stone cannot be perfectly protected unless this is done 
on all sides of the stone. Such coating does not in any way prevent the 
adhesion of the mortar if done either before or after it is put into the 
building. 

In the experiments made in the year 1861,1 was successful in prevent¬ 
ing the further decay of a building which has, up to this time, shown 
no sign of lamination or disintegration. This was done by the use of 
thoroughly boiled oil alone, applied with a brush duriug the warmest of 
our summer weather, when the stone was very hot. It was done twice 
at intervals of several years, and completely arrested the decay for the 
time being. It would have been much more effectual if the preparations 
mentioned above had been used. The application of such material does 
not change the surface of the stone. It was used almost exclusively 
to porous sandstones. It seems to form a sort of cement, similar to the 
organic binding material which is so common in many of the brown 
sandstones, but with the limestones this has not been found useful to 
any extent. It requires renewal so often, and changes the color of the 
stone so unpleasantly, that it has not been found of service with these 
stones. Such preparations are, however, of no use on stones of 1 what¬ 
ever character, where the decay has been produced by disintegration 
caused by the unequal expansion and contraction of the minerals form¬ 
ing the stone. Such disintegration is very slow in most stones. It has 
taken thousands of years to prodnce it in the granites of the obelisks of 
hot countries. It appears to be more rapid in cold ones; but it takes 
place with unerring certainty and regularity in all those rocks where the 
minerals composing them have different rates of expansion and con¬ 
traction. Here, as in the other case, there are cavities, but the filling of 
them does not arrest the decay, because it does not attack the cause. 
Nothing will arrest such decay, and no other decay in stone, except 
those caused by the air and water can be prevented by any appli¬ 
cation to its outside. It is therefore useless to attempt the preserva¬ 
tion of such rocks as granite, since the unequal expansion of the 
minerals which compose the rock cannot be arrested by filling the 
cracks. They are applicable only to stones from which something has 
been dissolved out, leaving cavities which can be filled from the outside. 
Water-proofing the surface of a stone has the effect of keeping the water 


out of pores that allow some of the constituents of the stone to be at¬ 
tacked, and is effectual in those stones only, which are either porous by 
nature, or have become so by the solution of parts of their ingredients; 
but it has absolutely no effect where there is any movement in the parti¬ 
cles which compose the stone, however this movement may be produced. 

No method of protecting the surface of the stone of the Houses of 
Parliament has, so far, been successful, excejDt where sulphur was dis¬ 
solved and added to the outside of the stone, and this rendered the stone 
for a very long time waterproof. It had remained so for twenty years 
up to August, 1884, when I examined it, and found that it had begun to 
laminate, the part of the stone containing the sulphur peeling off in thin 
scales. The experience of the repairs at the House of Commons is that 
most of the substances which are supposed to be useful, are absolutely 
useless, if not positively harmful to that stone; that almost the only one 
that has been successful at all on these stones has been sulphur 
dissolved in some compound, and applied to the stone so that the 
sulphur itself was precipitated in its pores. But even this does not seem 
to have entered very far, and has not been successful unless the stones 
put in place of those removed had been treated on all their six sides. 
In that case the stone has been preserved, but how long it will remain 
so it is impossible to tell. 

With dolomite, in which there is a large excess of lime, the only 
safety is to prevent the action of the carbonic acid contained in the air 
by water-proofing the outside of the stone. This can be less success¬ 
fully accomplished, because as the surfaces of attack are comparatively 
large, the action is from the outside, and but little pitting takes place, as 
the interlacing of the crystals makes cavities that never can penetrate 
far into the stone. Any coating applied to the outside will therefore be 
likely to wear off after a time, and thus leave fresh surfaces of attack, 
which do not at once become visible, as the surface is constantly kept 
clean by the rain and the wind. 

All the experiments made in Europe with the use of silicate of soda or 
water-glass upon any building stones, except those in which lime and 
magnesia were principal elements, have met with signal failure. The 
use of water-glass or siliceous material amounts to nothing on siliceous 
rocks. The silica becomes decomposed by exposure to the air, and forms 
a sand which drops off, and the caustic alkali is washed out, or remains 
behind to help the disintegration. The only things which have been 


58 


successful have been those that prevented the attack of the aeid gases 
or water from the surface or sides of the stone. 

Water-proofing is best done by a compound of paraffine, sulphur, 
and oil, applied to the stones before they are put into the building, or 
else frequently applied to the surface after they are put in. In sand¬ 
stones the sulphur is not necessary. Oil, with a certain amount of 
paraffine, may be used, providing the application is made hot. If, how¬ 
ever, it is made cold, the preserving material sinks but a little way into 
the stone. If the stone is heated from the outside, with the intent to 
bring the surface of the stone up to such a point that it will heat the 
paraffine, there is great danger that the stone will suffer more from the 
remedy than from the disease. 

There is no necessity for the decomposition of brownstone if the ma¬ 
terial is carefully selected. Every building that I have examined con¬ 
tains some stones that would last indefinitely, and if only those were 
selected from the quarry, which have a siliceous cement, there would be 
no necessity for waterproofing, and we should not have the rapid 
destruction in beautiful structures which is so common in countries 
where sandstones are used. The same kind of external water-proofing 
must also be done upon dolomite and limestone, but efforts in this direc¬ 
tion have been less successful than upon sandstones. Many methods 
for the prevention of disintegration have been tried, not only with no 
success, but with absolute failure. 







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